[FFmpeg-devel] [PATCH] cpu: Limit the number of auto threads in 32 bit builds

[Martin Storsjö]

Limit the returned value from av_cpu_count to sensible amounts
in 32 bit builds.

This chosen limit, 64, is somewhat arbitrary - a 32 bit process
is capable of creating much more than 64 threads. But in many
cases, multiple parts of the encoding pipeline (decoder, filters,
encoders) all create a pool of threads, auto sized according to the
number of cores.

In one failing test, the process had managed to create 506 threads
before a pthread_create call failed.

In the current set of fate tests, the filter-lavd-scalenorm test
seems to be the limiting factor; in a 32 bit build (arm linux,
running on an aarch64 kernel), it starts failing with an auto
thread count somewhere around 85. Therefore, pick the maximum
with some margin below this.

This fixes running fate without any manually set number of threads
in 32 bit builds on machines with huge numbers of cores.
---
 libavutil/cpu.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/libavutil/cpu.c b/libavutil/cpu.c
index 0035e927a5..094bd71d3d 100644
--- a/libavutil/cpu.c
+++ b/libavutil/cpu.c
@@ -233,6 +233,12 @@ int av_cpu_count(void)
     nb_cpus = sysinfo.dwNumberOfProcessors;
 #endif
 
+#if SIZE_MAX <= UINT32_MAX
+    // Avoid running out of memory/address space in 32 bit builds, by
+    // limiting the number of auto threads.
+    nb_cpus = FFMIN(nb_cpus, 64);
+#endif
+
     if (!atomic_exchange_explicit(&printed, 1, memory_order_relaxed))
         av_log(NULL, AV_LOG_DEBUG, "detected %d logical cores\n", nb_cpus);
 
-- 
2.25.1