分布式构建
分布式构建可以通过多台机器来分担本地的编译压力，加快构建速度。
NixOS 官方的 cache.nixos.org 中提供了绝大多数 X86_64 架构的缓存，因此对于普通 X86_64 的用户，一般不需要分布式构建。
分布式构建只在没有缓存可用的场景下才有较大应用价值，主要有这几种应用场景：
- RISC-V 或 ARM64 架构的用户（尤其是 RISC-V），因为官方缓存仓库中这两个架构的缓存很少，导致经常需要大量本地编译。
- 对系统进行大量定制的用户，因为官方缓存仓库中的 packages 都是默认配置，如果你改了构建参数，那么官方缓存就不适用了，这时候就需要本地编译。
- 比如嵌入式场景下往往对底层内核、驱动等有定制需求，导致需要本地编译。
配置分布式构建
官方没有详细文档讲这个，我在文末列出了一些建议阅读的参考文档，同时如下是我的分布式构建配置（一个 NixOS Module）：
nix
{ ... }: {
####################################################################
#
# NixOS's Configuration for Remote Building / Distributed Building
#
####################################################################
# set local's max-job to 0 to force remote building(disable local building)
# nix.settings.max-jobs = 0;
nix.distributedBuilds = true;
nix.buildMachines =
let
sshUser = "ryan";
# ssh key's path on local machine
sshKey = "/home/ryan/.ssh/ai-idols";
systems = [
# native arch
"x86_64-linux"
# emulated arch using binfmt_misc and qemu-user
"aarch64-linux"
"riscv64-linux"
];
# all available system features are poorly documentd here:
# https://github.com/NixOS/nix/blob/e503ead/src/libstore/globals.hh#L673-L687
supportedFeatures = [
"benchmark"
"big-parallel"
"kvm"
];
in
[
# Nix seems always give priority to trying to build remotely
# to make use of the local machine's high-performance CPU, do not set remote builder's maxJobs too high.
{
# some of my remote builders are running NixOS
# and has the same sshUser, sshKey, systems, etc.
inherit sshUser sshKey systems supportedFeatures;
# the hostName should be:
# 1. a hostname that can be resolved by DNS
# 2. the ip address of the remote builder
# 3. a host alias defined globally in /etc/ssh/ssh_config
hostName = "aquamarine";
# remote builder's max-job
maxJobs = 3;
# speedFactor's a signed integer
# but it seems that it's not used by Nix, takes no effect
speedFactor = 1;
}
{
inherit sshUser sshKey systems supportedFeatures;
hostName = "ruby";
maxJobs = 2;
speedFactor = 1;
}
{
inherit sshUser sshKey systems supportedFeatures;
hostName = "kana";
maxJobs = 2;
speedFactor = 1;
}
];
# optional, useful when the builder has a faster internet connection than yours
nix.extraOptions = ''
builders-use-substitutes = true
'';
# define the host alias for remote builders
# this config will be written to /etc/ssh/ssh_config
programs.ssh.extraConfig = ''
Host ai
HostName 192.168.5.100
Port 22
Host aquamarine
HostName 192.168.5.101
Port 22
Host ruby
HostName 192.168.5.102
Port 22
Host kana
HostName 192.168.5.103
Port 22
'';
# define the host key for remote builders so that nix can verify all the remote builders
# this config will be written to /etc/ssh/ssh_known_hosts
programs.ssh.knownHosts = {
# 星野 愛久愛海, Hoshino Aquamarine
aquamarine = {
hostNames = [ "aquamarine" "192.168.5.101" ];
publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDnCQXlllHoLX5EvU+t6yP/npsmuxKt0skHVeJashizE";
};
# 星野 瑠美衣, Hoshino Rubii
ruby = {
hostNames = [ "ruby" "192.168.5.102" ];
publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIE7n11XxB8B3HjdyAsL3PuLVDZxWCzEOUTJAY8+goQmW";
};
# 有馬 かな, Arima Kana
kana = {
hostNames = [ "kana" "192.168.5.103" ];
publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJ3dDLOZERP1nZfRz3zIeVDm1q2Trer+fWFVvVXrgXM1";
};
};
}缺陷
目前我观察到的问题有：
- 无法在构建时指定使用哪些主机，只能在配置文件中指定一个主机列表，然后 nix 会自动选择可用的主机。
- 在选择主机时，我发现 Nix 总是优先选择远程主机，而我本地主机的性能最强，这导致本地主机的 CPU 无法充分利用。
- 多机远程构建是以 Derivation 为单位的，因此在构建一些比较大的包时，其他机器可能会空闲很久，一直等这个大包构建完毕，这导致了资源的浪费。
- 在构建的 packages 较多并且可以并行执行时，可以轻松将所有主机的 CPU 都用上，这确实非常爽。