Re: [PATCH v2 3/6] util/oslib-posix: Don't create too many threads with small memory or little pages

[Pankaj Gupta]

> Let's limit the number of threads to something sane, especially that
> - We don't have more threads than the number of pages we have
> - We don't have threads that initialize small (< 64 MiB) memory
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  util/oslib-posix.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index 2ae6c3aaab..a1d309d495 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -40,6 +40,7 @@
>  #include <libgen.h>
>  #include "qemu/cutils.h"
>  #include "qemu/compiler.h"
> +#include "qemu/units.h"
>
>  #ifdef CONFIG_LINUX
>  #include <sys/syscall.h>
> @@ -529,7 +530,8 @@ static void *do_madv_populate_write_pages(void *arg)
>      return NULL;
>  }
>
> -static inline int get_memset_num_threads(int smp_cpus)
> +static inline int get_memset_num_threads(size_t hpagesize, size_t numpages,
> +                                         int smp_cpus)
>  {
>      long host_procs = sysconf(_SC_NPROCESSORS_ONLN);
>      int ret = 1;
> @@ -537,6 +539,12 @@ static inline int get_memset_num_threads(int smp_cpus)
>      if (host_procs > 0) {
>          ret = MIN(MIN(host_procs, MAX_MEM_PREALLOC_THREAD_COUNT), smp_cpus);
>      }
> +
> +    /* Especially with gigantic pages, don't create more threads than pages. 
> */
> +    ret = MIN(ret, numpages);
> +    /* Don't start threads to prealloc comparatively little memory. */
> +    ret = MIN(ret, MAX(1, hpagesize * numpages / (64 * MiB)));
> +
>      /* In case sysconf() fails, we fall back to single threaded */
>      return ret;
>  }
> @@ -546,7 +554,7 @@ static bool touch_all_pages(char *area, size_t hpagesize, 
> size_t numpages,
>  {
>      static gsize initialized = 0;
>      MemsetContext context = {
> -        .num_threads = get_memset_num_threads(smp_cpus),
> +        .num_threads = get_memset_num_threads(hpagesize, numpages, smp_cpus),
>      };
>      size_t numpages_per_thread, leftover;
>      void *(*touch_fn)(void *);

Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>