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https://github.com/ryan4yin/nixos-and-flakes-book
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146 lines
5.6 KiB
Markdown
146 lines
5.6 KiB
Markdown
# 分布式构建
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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。
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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。
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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：
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1. 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。
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2. 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。
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1. 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。
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## 配置分布式构建
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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）：
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```nix
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{ ... }: {
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####################################################################
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#
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# NixOS's Configuration for Remote Building / Distributed Building
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#
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####################################################################
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# set local's max-job to 0 to force remote building(disable local building)
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# nix.settings.max-jobs = 0;
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nix.distributedBuilds = true;
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nix.buildMachines =
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let
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sshUser = "ryan";
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# ssh key's path on local machine
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sshKey = "/home/ryan/.ssh/ai-idols";
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systems = [
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# native arch
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"x86_64-linux"
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# emulated arch using binfmt_misc and qemu-user
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"aarch64-linux"
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"riscv64-linux"
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];
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# all available system features are poorly documentd here:
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# https://github.com/NixOS/nix/blob/e503ead/src/libstore/globals.hh#L673-L687
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supportedFeatures = [
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"benchmark"
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"big-parallel"
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"kvm"
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];
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in
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[
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# Nix seems always give priority to trying to build remotely
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# to make use of the local machine's high-performance CPU, do not set remote builder's maxJobs too high.
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{
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# some of my remote builders are running NixOS
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# and has the same sshUser, sshKey, systems, etc.
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inherit sshUser sshKey systems supportedFeatures;
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# the hostName should be:
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# 1. a hostname that can be resolved by DNS
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# 2. the ip address of the remote builder
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# 3. a host alias defined globally in /etc/ssh/ssh_config
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hostName = "aquamarine";
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# remote builder's max-job
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maxJobs = 3;
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# speedFactor's a signed integer
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# but it seems that it's not used by Nix, takes no effect
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speedFactor = 1;
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}
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{
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inherit sshUser sshKey systems supportedFeatures;
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hostName = "ruby";
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maxJobs = 2;
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speedFactor = 1;
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}
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{
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inherit sshUser sshKey systems supportedFeatures;
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hostName = "kana";
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maxJobs = 2;
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speedFactor = 1;
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}
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];
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# optional, useful when the builder has a faster internet connection than yours
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nix.extraOptions = ''
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builders-use-substitutes = true
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'';
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# define the host alias for remote builders
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# this config will be written to /etc/ssh/ssh_config
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programs.ssh.extraConfig = ''
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Host ai
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HostName 192.168.5.100
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Port 22
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Host aquamarine
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HostName 192.168.5.101
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Port 22
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Host ruby
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HostName 192.168.5.102
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Port 22
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Host kana
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HostName 192.168.5.103
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Port 22
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'';
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# define the host key for remote builders so that nix can verify all the remote builders
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# this config will be written to /etc/ssh/ssh_known_hosts
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programs.ssh.knownHosts = {
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# 星野 愛久愛海, Hoshino Aquamarine
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aquamarine = {
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hostNames = [ "aquamarine" "192.168.5.101" ];
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publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDnCQXlllHoLX5EvU+t6yP/npsmuxKt0skHVeJashizE";
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};
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# 星野 瑠美衣, Hoshino Rubii
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ruby = {
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hostNames = [ "ruby" "192.168.5.102" ];
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publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIE7n11XxB8B3HjdyAsL3PuLVDZxWCzEOUTJAY8+goQmW";
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};
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# 有馬 かな, Arima Kana
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kana = {
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hostNames = [ "kana" "192.168.5.103" ];
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publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJ3dDLOZERP1nZfRz3zIeVDm1q2Trer+fWFVvVXrgXM1";
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};
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};
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}
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```
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## 缺陷
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目前我观察到的问题有：
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1. 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。
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2. 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。
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3. 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。
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1. 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。
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## References
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- [Distributed build - NixOS Wiki](https://nixos.wiki/wiki/Distributed_build)
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- [Document available system features - nix#7380](https://github.com/NixOS/nix/issues/7380)
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- [Distributed builds seem to disable local builds nix#2589](https://github.com/NixOS/nix/issues/2589)
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- [Offloading NixOS builds to a faster machine](https://sgt.hootr.club/molten-matter/nix-distributed-builds/)
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- [tests/nixos/remote-builds.nix - Nix Source Code](https://github.com/NixOS/nix/blob/713836112/tests/nixos/remote-builds.nix#L46)
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