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QThread/Linux & FreeBSD: replace the QVLA with a real VLA and bump max

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We don't need to use the heap, because this array won't get too big.
This commit raises the limit from 4096 logical processors on Linux to
1048576 (i.e., the square). 4096 is less than one system I've had access
to since I wrote this code in 2021 (64 sockets of hyperthreaded 60-core
Intel 4th Generation Xeon Scalable processors = 7680 logical
processors). The 4096 limit would also be reached by a "mere" 16 sockets
of 288-core Intel Xeon 6E processors.

I've also made it grow slightly faster by multiplying by 4 instead of 2.

Change-Id: I7c0d8b5e7809faba72c2fffdf99500868dfd7db4
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
This commit is contained in:
Thiago Macieira 2025-01-17 09:25:09 -08:00
parent 8500e73040
commit d3b5705c94
1 changed files with 16 additions and 11 deletions
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27
src/corelib/thread/qthread_unix.cpp
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@ -502,19 +502,24 @@ int QThread::idealThreadCount() noexcept
cores = (int)psd.psd_proc_cnt;
}
#elif (defined(Q_OS_LINUX) && !defined(Q_OS_ANDROID)) || defined(Q_OS_FREEBSD)
QT_WARNING_PUSH
# if defined(Q_CC_CLANG) && Q_CC_CLANG >= 1800
QT_WARNING_DISABLE_CLANG("-Wvla-cxx-extension")
# endif
// get the number of threads we're assigned, not the total in the system
QVarLengthArray<cpu_set_t, 1> cpuset(1);
int size = 1;
if (Q_UNLIKELY(sched_getaffinity(0, sizeof(cpu_set_t), cpuset.data()) < 0)) {
for (size = 2; size <= 4; size *= 2) {
cpuset.resize(size);
if (sched_getaffinity(0, sizeof(cpu_set_t) * size, cpuset.data()) == 0)
break;
constexpr qsizetype MaxCpuCount = 1024 * 1024;
constexpr qsizetype MaxCpuSetArraySize = MaxCpuCount / sizeof(cpu_set_t) / 8;
qsizetype size = 1;
do {
cpu_set_t cpuset[size];
if (sched_getaffinity(0, sizeof(cpu_set_t) * size, cpuset) == 0) {
cores = CPU_COUNT_S(sizeof(cpu_set_t) * size, cpuset);
break;
}
if (size > 4)
return 1;
}
cores = CPU_COUNT_S(sizeof(cpu_set_t) * size, cpuset.data());
size *= 4;
} while (size < MaxCpuSetArraySize);
QT_WARNING_POP
#elif defined(Q_OS_BSD4)
// OpenBSD, NetBSD, BSD/OS, Darwin (macOS, iOS, etc.)
size_t len = sizeof(cores);