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libstore/globals.cc: Automatically set cores based on cgroup CPU limit

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By default, Nix sets the "cores" setting to the number of CPUs which are
physically present on the machine. If cgroups are used to limit the CPU
and memory consumption of a large Nix build, the OOM killer may be
invoked.

For example, consider a GitLab CI pipeline which builds a large software
package. The GitLab runner spawns a container whose CPU is limited to 4
cores and whose memory is limited to 16 GiB. If the underlying machine
has 64 cores, Nix will invoke the build with -j64. In many cases, that
level of parallelism will invoke the OOM killer and the build will
completely fail.

This change sets the default value of "cores" to be
ceil(cpu_quota / cpu_period), with a fallback to
std:🧵:hardware_concurrency() if cgroups v2 is not detected.
This commit is contained in:
Alex Wied 2022-07-15 00:14:29 -04:00
parent fbd0a6c6e2
commit 1af5d798a4
1 changed files with 49 additions and 1 deletions
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50
src/libstore/globals.cc
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@ -11,6 +11,11 @@
#include <dlfcn.h>
#include <sys/utsname.h>
#if __linux__
#include <mntent.h>
#include <cmath>
#endif
#include <nlohmann/json.hpp>
@ -114,7 +119,50 @@ std::vector<Path> getUserConfigFiles()
unsigned int Settings::getDefaultCores()
{
return std::max(1U, std::thread::hardware_concurrency());
unsigned int concurrency = std::max(1U, std::thread::hardware_concurrency());
#if __linux__
FILE *fp = fopen("/proc/mounts", "r");
if (!fp)
return concurrency;
Strings cgPathParts;
struct mntent *ent;
while ((ent = getmntent(fp))) {
std::string mountType, mountPath;
mountType = ent->mnt_type;
mountPath = ent->mnt_dir;
if (mountType == "cgroup2") {
cgPathParts.push_back(mountPath);
break;
}
}
fclose(fp);
if (cgPathParts.size() > 0 && pathExists("/proc/self/cgroup")) {
std::string currentCgroup = readFile("/proc/self/cgroup");
Strings cgValues = tokenizeString<Strings>(currentCgroup, ":");
cgPathParts.push_back(trim(cgValues.back(), "\n"));
cgPathParts.push_back("cpu.max");
std::string fullCgPath = canonPath(concatStringsSep("/", cgPathParts));
if (pathExists(fullCgPath)) {
std::string cpuMax = readFile(fullCgPath);
std::vector<std::string> cpuMaxParts = tokenizeString<std::vector<std::string>>(cpuMax, " ");
std::string quota = cpuMaxParts[0];
std::string period = trim(cpuMaxParts[1], "\n");
if (quota != "max")
concurrency = std::ceil(std::stoi(quota) / std::stof(period));
}
}
#endif
return concurrency;
}
StringSet Settings::getDefaultSystemFeatures()