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43 lines
3.8 KiB
Markdown
43 lines
3.8 KiB
Markdown
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# Tuning Cores and Jobs
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Nix has two relevant settings with regards to how your CPU cores will be
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utilized: [???](#conf-cores) and [???](#conf-max-jobs). This chapter
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will talk about what they are, how they interact, and their
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configuration trade-offs.
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- [???](#conf-max-jobs)
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Dictates how many separate derivations will be built at the same
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time. If you set this to zero, the local machine will do no builds.
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Nix will still substitute from binary caches, and build remotely if
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remote builders are configured.
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- [???](#conf-cores)
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Suggests how many cores each derivation should use. Similar to `make
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-j`.
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The [???](#conf-cores) setting determines the value of
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NIX\_BUILD\_CORES. NIX\_BUILD\_CORES is equal to [???](#conf-cores),
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unless [???](#conf-cores) equals `0`, in which case NIX\_BUILD\_CORES
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will be the total number of cores in the system.
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The maximum number of consumed cores is a simple multiplication,
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[???](#conf-max-jobs) \* NIX\_BUILD\_CORES.
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The balance on how to set these two independent variables depends upon
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each builder's workload and hardware. Here are a few example scenarios
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on a machine with 24 cores:
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| [???](#conf-max-jobs) | [???](#conf-cores) | NIX\_BUILD\_CORES | Maximum Processes | Result |
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| --------------------- | ------------------ | ----------------- | ----------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| 1 | 24 | 24 | 24 | One derivation will be built at a time, each one can use 24 cores. Undersold if a job can’t use 24 cores. |
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| 4 | 6 | 6 | 24 | Four derivations will be built at once, each given access to six cores. |
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| 12 | 6 | 6 | 72 | 12 derivations will be built at once, each given access to six cores. This configuration is over-sold. If all 12 derivations being built simultaneously try to use all six cores, the machine's performance will be degraded due to extensive context switching between the 12 builds. |
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| 24 | 1 | 1 | 24 | 24 derivations can build at the same time, each using a single core. Never oversold, but derivations which require many cores will be very slow to compile. |
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| 24 | 0 | 24 | 576 | 24 derivations can build at the same time, each using all the available cores of the machine. Very likely to be oversold, and very likely to suffer context switches. |
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Balancing 24 Build Cores
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It is up to the derivations' build script to respect host's requested
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cores-per-build by following the value of the NIX\_BUILD\_CORES
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environment variable.
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