# Fornax's Guide To Ridiculously Fast Ethernet

- [Introduction](#introduction)
- [Sysctls](#sysctls)
  - [net.ipv4.tcp_congestion_control](#net-ipv4-tcp-congestion-control)
  - [net.core.default_qdisc and txqueuelen](#net-core-default-qdisc-and-txqueuelen)
  - [net.ipv4.tcp_shrink_window](#net-ipv4-tcp-shrink-window)
  - [net.ipv4.tcp_{w,r}mem](#net-ipv4-tcp-w-r-mem)
  - [net.ipv4.tcp_mem](#net-ipv4-tcp-mem)
- [Network Interface Cards](#network-interface-cards)
- [ethtool](#ethtool)
  - [Channels (ethtool -l)](#channels-ethtool-l)
  - [Ring buffers (ethtool -g)](#ring-buffers-ethtool-g)
  - [Interrupt Coalescing (ethtool -c)](#interrupt-coalescing-ethtool-c)
- [BIOS](#bios)
  - [NUMA Nodes per socket](#numa-nodes-per-socket)
  - [SMT Control](#smt-control)
  - [IOMMU](#iommu)
- [Reverse proxy](#reverse-proxy)
- [HTTP/2 or QUIC?](#http-2-or-quic)
- [Operating system](#operating-system)
- [Kernel](#kernel)
- [That's all, folks!](#that-s-all-folks)

## Introduction

If you are one of the lucky few who has a fast enough connection, you might have
just downloaded a 5 GB file in 10 seconds and wondered how that is even
possible. Well, it took a lot of effort to get there.

When I first ordered a 100 GbE server I expected things to just work. Imagine my
surprise when the server crashed when serving at just 20 Gigabit.

Then I expected my server host to be able to help with the performance problems.
Spoiler alert: They could not help me.

That's where my journey into the rabbit hole of network performance started. I
paid good money for that 100 Gigabit connection and I'll be damned if I can't
use all of it. I'm getting to the bottom of this no matter how long it takes...

It took me well over a year to figure out all the details of high speed
networking. My good friend [Jeff
Brandt](https://www.linkedin.com/in/jeff-brandt-51b2a65/) (who has been hosting
pixeldrain for nearly ten years now) was able to point me in the right direction
by explaining the basics and showing me some sysctls and ethtool commands which
might affect performance. That was just the entrance of the rabbit hole though,
and this one carried on deep. After about a year of trial and error pixeldrain
can finally serve files at 100 Gigabit per second.

Below is a summary of everything I discovered during my year of reading NIC
manuals, digging through the kernel sources, running profilers, patching the
kernel, learning about CPU topology and TCP inner workings.

## Sysctls

When looking into network performance problems the `sysctl`s are usually the
first thing you get pointed at. There is **a ton** of conflicting information
online about which sysctls do what and what to set them to.

Sysctls are not persistent through reboots, add these lines to
`/etc/sysctl.conf` to apply them at startup.

Through experimentation and kernel recompilation I finally settled on these
values:

### net.ipv4.tcp_congestion_control

You might have heard of BBR. Google's new revolutionary congestion control
algorithm. You might have heard conflicting information about how good it is. I
have extensively tested all congestion controls in the kernel and I can say
without a doubt that BBR is the best, by far! BBR is the only algo which does
not absolutely tank your transfer rate when a packet is lost.

TCP BBR was merged into the kernel at version 4.9. I know the sysctl says ipv4,
but it works for IPv6 as well.

`net.ipv4.tcp_congestion_control=bbr`

### net.core.default_qdisc and txqueuelen

The qdisc (queuing discipline) is another param which gets mentioned often. The
qdisc orders packets which are queued so they can be sent in the most efficient
order possible. The thing is, when you're sending at 100 Gbps then queuing is
completely irrelevant, the network is rarely the bottleneck here.

Google used to require `fq` with `bbr`, but that requirement has been dropped. I
suggest you use something minimal and fast. How about `pfifo_fast`, it has fast
in the name, must be good, right? This is actually already the default on Linux
nowadays, so there's not really a need to change it.

`net.core.default_qdisc=pfifo_fast`

A queue must have a size though. Linux gives the network queues a size of 1000
packets by default. As we'll learn later, a thousand packets is really not a lot
when running at 100 Gbps. When the queue is full the kernel will actually drop
packets, which is absolutely not what we want. So we increase the queue length
to 10000 packets instead:

`ip link set $INTERFACE txqueuelen 10000`

### net.ipv4.tcp_shrink_window

This sysctl was developed by Cloudflare. The patch was merged into Linux 6.1. If
you are on an older kernel version than 6.1 you will need to manually apply [the
patches](https://github.com/cloudflare/linux/) and compile the kernel on your
machine. Without this patch the kernel will waste so much time and memory on
buffer management that by the time you reach 100 Gigabit the kernel won’t even
have time to run your app anymore.

Cloudflare has an extensive writeup about the problem this sysctl solves here:
[Unbounded memory usage by TCP for receive buffers, and how we fixed
it](https://blog.cloudflare.com/unbounded-memory-usage-by-tcp-for-receive-buffers-and-how-we-fixed-it/)

This sysctl makes sure that TCP buffers are shrunk if they are larger than they
need to be. Without this sysctl your buffers will just continue to grow until
memory runs out! Before I discovered this patch my servers would regularly run
out of memory during peak load, and these are servers with a **TeraByte of
RAM**! After applying the patches (and compiling the kernel, because the patches
were not merged yet back then) memory usage from TCP buffers was reduced by 80%
on my systems. And performance has improved considerably. This patch is so
crucial for performance that it boggles my mind that it's not enabled by
default. It's even described in the [TCP
spec](https://www.rfc-editor.org/rfc/rfc7323#section-2.4), it's standardized
behaviour. If you're a kernel or systemd developer, please consider just turning
this on by default instead of hiding it behind a toggle.

`net.ipv4.tcp_shrink_window=1`

Cloudflare has some other sysctls as well, but those focus more on latency than
throughput. You can find them here: [Optimizing TCP for high WAN throughput
while preserving low
latency](https://blog.cloudflare.com/optimizing-tcp-for-high-throughput-and-low-latency/).
The `net.ipv4.tcp_collapse_max_bytes` sysctl they write about here was never
merged into the kernel. But while it does improve latency a bit, it's not that
important for throughput.

### net.ipv4.tcp_{w,r}mem

These variables dictate how much memory can be allocated for your send and
receive buffers. The send and receive buffers are where TCP packets are stored
which are not yet acknowledged by the peer. The required size of these buffers
depends on your [Bandwidth-Delay Product
(BDP)](https://en.wikipedia.org/wiki/Bandwidth-delay_product). This concept is
crucial to understand. If you set the TCP buffers too small it will literally
put a speed limit on your connection.

First let's go over how TCP sends data. TCP can retransmit packets if the client
did not receive them. To do this TCP needs to keep all the data it sends to the
client in memory until the client acknowledges (ACK) that it has been properly
received. The acknowledgment takes one round trip to the client and back.

Let's say you want to send a file from Amsterdam to Tokyo. The server sends the
first packet, 130ms later the client in Tokyo receives the data packet. The
client then sends ACK to tell the server that the packet was properly received,
the ACK takes 130ms to arrive back in Amsterdam. Only now can the server remove
the packet from memory. The whole exchange took 260ms.

Now let's say we want to send files at 10 Gigabit. 10 Gigabit is 1250 MB. We
multiply the number of bytes we want to send per second by the number of seconds
it takes to get back the ACK. That's `1250 MB * 0.260 s = 325 MB`. Now we know
that our buffer needs to be at least 325 MB to reach a speed of 10 Gigabit over
a 260ms round trip.

The kernel also stores some other TCP-related stuff in that memory, and we also
need to account for packet loss which causes packets to be stored for a longer
time. I also don't want the speed to be limited to 10 Gbps, we're running a 100
GbE NIC after all. For this reason pixeldrain servers use a maximum buffer size
of 1 GiB.

```
net.ipv4.tcp_wmem='4096 65536 1073741824'
net.core.wmem_max=1073741824
net.ipv4.tcp_rmem='4096 65536 1073741824'
net.core.rmem_max=1073741824
```

The three values in the wmem and rmem are the minimum buffer size, the default
buffer size and the maximum buffer size. The pixeldrain server application uses
64k reusable buffers (with [sync.Pool](https://pkg.go.dev/sync#Pool)) all over
the codebase. For this reason we initialize the window size at 64k as well.

### net.ipv4.tcp_mem

We just configured the buffer sizes, what's this for then? Well... we can tune
TCP buffers per connection all we want, but all that is for nothing if the
kernel still limits the TCP buffers globally.

This sysctl configures how much system memory can be used for TCP buffers. On
boot these values are set based on available system memory, which is good. But
by default it only uses like 5% of the memory, which is not even close to
enough. We need to pump those numbers way up to get anywhere near the speed that
we want.

tcp_mem is defined as three separate values. These values are in numbers of
memory pages. A memory page is usually 4096B. Here is what these three values mean:

 * `low`: When TCP memory is below this threshold then TCP buffer sizes are not
   limited.
 * `pressure`: When the TCP memory usage exceeds this threshold it will try to
   shrink some TCP buffers to free up memory. It will keep doing this until
   memory usage drops below `low` again. Shrinking TCP buffers takes a lot of
   CPU time, and during this time no data is sent to the client. You don't want
   to set `low` and `pressure` too far apart.
 * `high`: The TCP system can't allocate more than this number of pages. If this
   limit is reached and a new TCP session is opened it will not be able to
   allocate any memory. Needless to say this is terrible for performance.

After a lot of experimentation with these values I have come to the conclusion
that the best values for these parameters are 60% of RAM, 70% of RAM and 80% of
RAM. This will use most of the RAM for TCP buffers if needed, but also leaves
plenty for your applications.

I set these values dynamically per host with Ansible:

```yaml
{{noescape `- name: configure tcp_mem
  sysctl:
    name: net.ipv4.tcp_mem
    value: "{{ (mempages|int * 0.6)|int }} {{ (mempages|int * 0.7)|int }} {{ (mempages|int * 0.8)|int }}"
    state: present
  vars:
    mempages: "{{ ansible_memtotal_mb * 256 }}" # There are 256 mempages in a MiB`}}
```

## Network Interface Cards

There are lots of NICs to choose from. From my testing every NIC seems to behave
differently. The only NIC types I have had any luck with are ConnectX-5 and
ConnectX-6. Intel's E810 NICs are also not terrible, but Nvidia cards seem to
fare much better with high connection counts. I currently have two servers with
E810 cards and two servers with ConnectX-6 cards. The E810 cards are usually the
first to crap out during a load peak. NICs are just fickle beasts overall. I
don't know if my experiences are actually related to the quality of the cards,
or just bad luck with faulty hardware.

Often you see advice to install a proprietary driver for your NIC. Don't do
that. In my experience that has only caused problems. Nvidia's NIC drivers are
just as shitty as their video drivers. They will break kernel updates and
generally make your life miserable. The drivers in the Linux kernel are good and
well maintained. You don't need to taint your kernel with some scary proprietary
blob.

Upgrading the firmware for your NIC can be a good idea, if you can figure out
how, that is. Nvidia's tools for upgrading firmware are a huge hassle to work
with and the documentation is outdated and scarce.

## ethtool

Ethtool is a program which you can use to configure your network card. There is
lots of stuff to configure here, but there are only three settings which really
matter.

Ethtool needs your network interface name for every operation. In this guide we
will refer to your interface name as `$INTERFACE`. You can get your interface
name from `ip a`.

Ethtool options are not persistent through reboots. And there's no configuration
file to put them in either. So you'll need to put them in a script which runs
somewhere in the boot process somehow.

### Channels (ethtool -l)

The channels param configures how many CPU cores will communicate with the NIC.
You generally want this number to be equal to the number of CPU cores you have,
that way the load will be evenly spread across your CPU. If you have more CPU
cores than your NIC supports you can try turning multithreading off in the BIOS.
Or just accept that only a portion of your cores will communicate with the NIC,
it's not that big of a problem.

If you are running on a multi-CPU platform you only want one CPU to communicate
with the NIC. Distributing your channels over multiple CPUs will cause cache
thrashing which absolutely tanks performance. Many of pixeldrain's server are
dual CPU, where one CPU runs the pixeldrain software and the other only
communicates with the NIC. Buying a $10k CPU just to talk to a NIC is a bit
wasteful, I recommend just using one CPU if you have the choice.

Your NIC will usually configure the channels correctly on boot, so in most of
the cases you don't need to change anything here. You can query the settings
with `ethtool -l $INTERFACE` and update the values like this: `ethtool -L
$INTERFACE combined 63`.

### Ring buffers (ethtool -g)

The ring buffers are portions of RAM where the NIC stores your IP packets before
they are sent out to the network (tx) or sent to the CPU (rx). Increasing the
ring buffer sizes can increase network latency a little bit because more packets
are getting buffered before being sent out to the network. But again, at 100 GbE
this happens so fast that the difference is in the order of microseconds, that
makes absolutely no difference to us. We just want to move as much data as
possible in as little time as possible.

If we can buffer more packets then it means we can transfer more data in bulk
with every clock cycle. So we simply set this to the maximum. For Mellanox cards
the maximum is usually `8192`, but this can vary. Check the maximum values for
your card with `ethtool -g $INTERFACE`.

Set the receive and send buffers to 8192 packets: `ethtool -G $INTERFACE rx 8192
tx 8192`

### Interrupt Coalescing (ethtool -c)

The NIC can't just write your packets to the CPU and expect it to do something
with them. Your CPU needs to be made aware that there is new data to process.
That happens with an interrupt. Ethtool's interrupt coalescing values tell the
NIC when and how to send interrupts to the CPU. This is a delicate balance. We
don't want to interrupt the CPU too often, because then it won't be able to get
any work done. That's like getting a new ping in team chat every half hour, how
are you supposed to concentrate like that? But if we set the interrupt rate too
slow, the NIC won't be able to send all packets in time before the buffers fill
up.

The interrupt coalescing options vary a lot per NIC type.. These are the ones
which are present on my ConnectX-6 Dx: `rx-usecs`, `rx-frames`, `tx-usecs`,
`tx-frames`, `cqe-mode-rx`, `cqe-mode-tx`. I'll explain what these are:

 * `rx-usecs`, `tx-usecs`: These values dictate how often the NIC interrupts the
   CPU to receive packets `rx` or send packets `tx`. The value is in
   microseconds. The SI prefix for micro is µ, but for convenience they use the
   letter u here. A microsecond is one-millionth of a second.
 * `rx-frames`, `tx-frames`: Like the values above this defines how often the
   CPU is interrupted, but instead of interrupting the CPU at a fixed interval
   it interrupts the CPU when a certain number of packets are in the buffer.
 * `cqe-mode-rx`, `cqe-mode-tx`: These options enable packet compression in the
   PCI bus. This is handy if your PCI bus is a bottleneck, like when your 100G
   NIC is plugged into 4x PCI 4.0 lanes, which only has 7.88 GB/s bandwidth. In
   most cases it's best to leave these at the default value.
 * `adaptive-rx`, `adaptive-tx`: These values tell the NIC to calculate its own
   interrupt timings. This disregards the values we configure ourselves. The
   timings calculated by the NIC often prefer low latency over throughput and
   can quickly overwhelm the CPU with interrupts. So for our purposes this needs
   to be disabled.

So what are good values for these? Well, we can do some math here. Our NIC can
send 100 Gigabits per second. That's 12.5 GB. A network packet is usually 1500
bytes. This means that we need to send 8333333 packets per second to reach full
speed. Our ring buffer can hold 8192 packets, so if we divide by that number we
learn that we need to send 1017 entire ring buffers per second to reach full
speed.

Waiting for the ring buffer to be completely full is probably not a good idea,
since then we can't add more packets until the previous packets have been copied
out. So we want to be able to empty the ring buffer twice. That leaves us with
2034 ring buffers per second. Now convert that buffers per second number to µs
per buffer: `1000000 / 2034 = 492µs`, we land on a value of 492µs per interrupt.
This is our ceiling value. Higher than this and the buffers will overflow. But
492µs is nearly half a millisecond, that's an eternity in CPU time. That's high
enough that it might actually make a measurable difference in packet latency. So
we opt for a more sane value of 100µs instead. That still gives the CPU plenty
of time to do other work in between interrupts. A 3 GHz CPU core will be able to
perform about 30000 calculations inbetween each interrupt. At the same time it's
low enough to barely make a measurable difference in latency, at most a tenth of
a millisecond.

As for the `{rx,tx}-frames` variables. We just spent all that time calculating
the ideal interrupt interval, I don't really want the NIC to start interrupting
my CPU when it's not absolutely necessary. So we use the maximum ring buffer
value here: `8192`. Your NIC might not support such high coalescing values. You
can also try setting this to `4096` or `2048` if you notice problems.

That leaves us with this configuration:

```
ethtool -C $INTERFACE adaptive-rx off adaptive-tx off \
		rx-usecs 100 tx-usecs 100 \
		rx-frames 8192 tx-frames 8192
```

Tip: If you want to see how much time your CPU is spending on handling
interrupts, go into `htop`, then to Setup (F2) and enable "Detailed CPU time"
under Display options. The CPU gauge will now show time spent on handling
interrupts in purple. Press F10 to save changes.

## BIOS

Not even the BIOS is safe from our optimization journey. If fact, some of the
most important optimizations must be configured here.

### NUMA Nodes per socket

Big CPUs with lots of cores often segment their memory into NUMA nodes. These
smaller nodes get exclusive access to a certain portion of RAM and don't have to
contend over memory access with the other NUMA nodes. This can improve your
performance... if your software supports it well. But from my testing the setup
of one NIC queue per core does not combine well with having multiple NUMA nodes.
The fact that I use Go, which does not have a NUMA aware scheduler as far as I
know, probably does not help either. For these reasons I prefer to set `NUMA
nodes per socket` to `NPS1`.

Some AMD BIOSes also have an option called `ACPI SRAT L3 Cache as NUMA Domain`.
This will create NUMA nodes based on the L3 cache topology, *even if you
explicitly disabled NUMA in the memory addressing settings*. To fix this set
`ACPI SRAT L3 Cache as NUMA Domain` to `Disabled`.

### SMT Control

Multithreading (or Hyperthreading, on Intel) can be a performance booster, but
it can also be a performance bottleneck. If you have a CPU with a lot of cores,
like AMD's Epyc lineup, then disabling SMT can be a good way to improve per-core
performance.

Most apps have no way to effectively use hundreds of CPU threads. At some point
adding more threads will only consume more memory and CPU cycles just because
they kernel scheduler, memory controller and your language runtime have to
manage all those threads. This can cause huge amounts of overhead. My rule of
thumb: If you have 64 or more cores: `SMT OFF`

### IOMMU

The [Input-output memory management
unit](https://en.wikipedia.org/wiki/Input%E2%80%93output_memory_management_unit)
is a CPU component for virtualizing your memory access. This can be useful if
you run a lot of VMs for example. You know what it's also good for? **Completely
destroying NIC performance**.

A high end NIC needs to shuffle a lot of data over the PCI bus. A 100 GbE NIC in
full duplex can reach up to 25 GB/s! When the IOMMU is enabled it means that all
the data that the NIC sends/receives needs to go through the IOMMU first before
it can go into RAM. This adds a little bit of latency. When you are running a
high end NIC in your PCI slot, then the added latency makes sure that your NIC
will **never ever get anywhwere near the advertised speed**. In some cases the
overhead is so large that the NIC will effectively drop off the PCI bus,
immediately crashing your system once it gets only slightly overloaded. Yes,
really, I have seen this happen.

Seriously, if you have a high end NIC plugged into your PCI slot and you have
the IOMMU enabled. **You might as well plug a goddamn brick into your PCI
slot**, because that's about as useful as your expensive NIC will be.

It took me way too long to find this information. The difference between IOMMU
off and on is night and day. I am actually **furious** that it took me so long
to discover this. I spent *weeks* pulling hair out of my head trying to figure
out why my NIC was locking up whenever I tried to put any real load on it. All
the NIC tuning guides I could find talk about tweaking little ethtool params,
installing drivers, updating firmware and useles crap like that, the IOMMU was
completely omitted in every one of them. I was getting so desperate with my
terrible NIC performance that I just started flipping toggles in the BIOS to see
if anything made a difference. If you have any idea how long it takes to reboot
a high end server system you know how tedious this is. That's how I discovered
that the IOMMU was the source of **all my problems**.

Ugh, just thinking about all the time I wasted because because nobody told me to
just turn the IOMMU off gets my blood boiling. That's why I am writing this
guide, I want to spare you the suffering.

So yea... `AMD CBS > NBIO Common Options > IOMMU > Disabled` ...AND STAY DOWN!

I also just turn off anything related to virtualization nowadays. Having
virtualization options enabled when you are not running VMs is a waste of
resources. No worries, docker is not virtualization, it's just namespacing,
nothing virtual about that. And if you are running VMs.. well, consider bare
metal. It's really not that scary and there is lots of performance to be gained.

You can verify that your IOMMU is disabled with this command `dmesg | grep
iommu`. Your IOMMU is disabled if it prints something along the lines of:

```
[    1.302786] iommu: Default domain type: Translated
[    1.302786] iommu: DMA domain TLB invalidation policy: lazy mode
```

If you see more output than that, you need to drop into the BIOS and nuke that
shit immediately.

One little caveat is that Linux requires the IOMMU to support more than 255 CPU
threads. So if you have 256 threads and the IOMMU is turned off one of your
threads will be disabled. So once again I will repeat my rule of thumb with
regards to multithreading: If you have 64 or more cores: `SMT OFF`

## Reverse proxy

A lot of sites run behind a reverse proxy like nginx or Caddy. It seems to be an
industry standard nowadays. People are surprised when they learn that pixeldrain
does not use one of the standard web servers.

As it turns out, 100 Gigabit per second is a lot of data. It takes a
considerable amount of CPU time to churn through that much data, so ideally you
want to touch it as few times as you can. At this scale playing hot potato with
your HTTP requests is a really bad idea.

A big bottleneck with networking on Linux is copying data across the kernel
boundary. The kernel always needs to copy your buffers because userspace is
dirty, ew, would not want to share memory with that. When you are running a
reverse proxy every HTTP request is effectively crossing the kernel boundary
*six times*. Let's assume we're running nginx here, the client sends a request
to the server. The kernel copies the request body from kernel space to nginx's
listener (from kernel space to userspace), nginx opens a request to your app and
copies the body the to localhost TCP socket (back to kernel space). The kernel
sends the body to your app's listener on localhost (now it's in userspace
again). And then the response body follows the same path again. Request: NIC ->
kernel -> userspace -> kernel -> userspace. Response: userspace -> kernel ->
userspace -> kernel -> NIC. That's crazy inefficient.

That's why pixeldrain just uses Go's built in HTTP server. Go's HTTP server is
very complete. Everything you need is there:

 * [Routing](https://github.com/julienschmidt/httprouter)
 * [TLS (for HTTPS)](https://pkg.go.dev/crypto/tls)
 * HTTP/2
 * Even a [reverse
   proxy](https://pkg.go.dev/net/http/httputil#NewSingleHostReverseProxy) if
   you're into that kinda stuff

The only requirement is that your app is written in Go. Of course other
languages also have libraries for this.

Zero-downtime restarts are a bit tricky. Luckily the geniuses tinkering away at
the Linux kernel every day made something neat for us. It's called
`SO_REUSEPORT` (Wow! Catchy name!). By putting this socket option on your TCP
listener you allow future instances of your server process to listen on the same
port at the same time. By doing this your upgrades become really quite simple:

1. Upload new server executable to the server.
2. Start the new executable up.
3. When everything is initialized it starts listening on the same port as the
   previous process using `SO_REUSEPORT`.
4. After the listener is installed we signal to the old server process (which is
   still running at this point) that it can start shutting down. The listener is
   closed and the active HTTP requests are gracefully completed.
5. Once the old listener is closed all new requests will go to the new process
   and the upgrade is complete.

Now there may be one question on your mind: How do I signal to the previous
process that the new process has finished initializing? I have just the thing
for you. [This handy-dandy library that I
made](https://github.com/Fornaxian/zerodown). I use it for pixeldrain and it
works like a charm. Your software updates are just one `SIGHUP` away from being
deployed.

## HTTP/2 or QUIC?

HTTP/2 and QUIC (HTTP/3) are new revisions of the HyperText Transfer Protocol.
HTTP/2 introduces multiplexing which significantly reduces handshake latency.
HTTP/1.1 will open a separate TCP session for each file it needs to request,
HTTP/2 opens one connection instead and uses framing to send multiple requests
at the same time instead, this allows the connection to ramp up to a higher
speed and quicker. This goes hand in hand with the BBR congestion control
algorithm which also significantly reduces connection ramp-up time. The result
is 60% faster loading times for web pages on average.

HTTP/2 is trivially enabled in the Go HTTP server. Simply add `NextProtos =
[]string{"h2"}` to your `tls.Config` and it's good to go. An annoying
implementation detail is that Go's HTTP/2 server throws completely different
errors than HTTP/1.1, so you will have to redo all your error handling. To make
matters worse, HTTP/2's errors are not exported by the `http` package, so you
have to resort to string searching to catch these errors.. 😒.

Then along comes HTTP/3, also known as QUIC. HTTP/3 throws everything we just
did out of the window and uses UDP instead. It moves all the buffer management
and congestion control to userspace. Sure, you get more control that way, but
that's really only useful if you're Google. I tried the most popular HTTP/3
server implementation for Go, and it struggled to even reach half of the
throughput I got with HTTP/2. Sure, latency is lower, but that's not that useful
to me when the most important part of my site stops functioning. Sure, TCP is
not perfect, but it's better than having to do everything yourself.

To summarize, if you only care about throughput: HTTP/2 👍 HTTP/3 👎 (for now)

## Operating system

Choose something up-to-date, lightweight and minimalist. Pixeldrain used to run
on Ubuntu because I was familiar with it, but over time Ubuntu server got more
bloated and heavy. Unnecessary stuff was being added with each new release
(looking at you snapd), and I just didn't want to deal with that. Eventually I
switched to Debian.

Debian is so much better than Ubuntu. After booting it for the first time there
will only be like 10 processes running on the system, just the essentials. It
really is a clean sandbox waiting for you to build a castle in it. It might take
some getting used to, but it will definitely pay off.

Anyway, that's just my opinion. In reality you can pick any distro you like. It
does not really matter that much. Just keep in mind that some distro's ship
newer kernels than others, and that's really quite important as we will learn in
the next paragraph.

## Kernel

You need to run at least kernel 6.1, because of the `net.ipv4.tcp_shrink_window`
sysctl. But generally, **newer is better**. There are dozens of engineers from
Google, Cloudflare and Meta tinkering away at the Linux network stack every day.
It gets better with every release, really, the pace is staggering.

But doesn't Debian ship quite old kernel packages? (you might ask) Yes... kinda.
By using [this guide](https://wiki.debian.org/HowToUpgradeKernel) you can
upgrade your kernel version to the `testing` or even the `experimental` branch
while keeping the rest of the OS the same.

On the [Debian package tracker](https://tracker.debian.org/pkg/linux) you can
see which kernel version ships in which repository. This is useful for picking
which repo you want to use for your kernel updates. Pixeldrain gets its kernel
updates from the `testing` branch. These are kernels which have been declared
stable by the kernel developers and are generally safe to use.

Keep an eye on the [Phoronix Linux Networking
blog](https://www.phoronix.com/linux/Linux+Networking) for new kernel features.
Pretty much every kernel version that comes out boasts about huge network
performance wins. I'm personally waiting for Kernel 6.8 to come out. They are
promising a 40% TCP performance boost. Crazy!

## That's all, folks!

**Behold.. One hundred gigabits per second!**

![nload showing 85 Gbps](/res/img/100gbps.webp)

Actually my nload seems to cap out at around 87 Gbps.. there's probably some
overhead somewhere. It's close though.

I hope this guide was useful to you. I wish I had something like this when I
started out. I could have quite literally saved me months of time. Then again,
chasing 100 Gigabit is one of the most educative challenges I have ever faced. I
have learned so much about Linux's structure, kernel performance profiling, CPU
architecture, the PCIe bus and tons of other things that I would never have
known if I did not go down this rabbit hole. And I have a feeling the journey is
not over. I will always have this urge to get the absolute most out of my
servers. I'm paying for the whole CPU and I'm going to use the whole CPU after
all.

Anyway, check out [Pixeldrain](/) if you like, it's the fastest way to transfer
files across the web. And I'm working on a [cloud storage](/filesystem) offering
as well. It has built in rclone and FTPS support. Pixeldrain also has a built in
[speedtest](/speedtest) which you can use to see the fruits of my labour. The
source for this document is available in markdown format on [my
GitHub](https://github.com/Fornaxian/pixeldrain_web/blob/master/res/include/md/100_gigabit_ethernet.md).

Follow me on [Mastodon](https://mastodon.social/@fornax),
[Twitter](https://twitter.com/Fornax96), join our
[Discord](https://discord.gg/pixeldrain), et cetera et cetera
