Re: Requesting review about optimizing large guest start up time

[Paolo Bonzini <pbonzini@redhat.com>]

I queued the patch, thanks!

I simplified a bit the computation of the number of pages per thread, like this:

    numpages_per_thread = numpages / memset_num_threads;
    leftover = numpages % memset_num_threads;
    for (i = 0; i < memset_num_threads; i++) {
        memset_thread[i].addr = addr;
        memset_thread[i].numpages = numpages_per_thread + (i < leftover);
        memset_thread[i].hpagesize = hpagesize;
        ...

This removes the need for a separate array and function.

Thanks,

Paolo

On 11/02/20 13:08, bauerchen(陈蒙蒙) wrote:
> From c882b155466313fcd85ac330a45a573e608b0d74 Mon Sep 17 00:00:00 2001
> From: bauerchen <bauerchen@tencent.com>
> Date: Tue, 11 Feb 2020 17:10:35 +0800
> Subject: [PATCH] Optimize: large guest start-up in mem-prealloc
> MIME-Version: 1.0
> Content-Type: text/plain; charset=utf-8
> Content-Transfer-Encoding: 8bit
> 
> [desc]:
>     Large memory VM starts slowly when using -mem-prealloc, and
>     there are some areas to optimize in current method;
> 
>     1、mmap will be used to alloc threads stack during create page
>     clearing threads, and it will attempt mm->mmap_sem for write
>     lock, but clearing threads have hold read lock, this competition
>     will cause threads createion very slow;
> 
>     2、methods of calcuating pages for per threads is not well;if we use
>     64 threads to split 160 hugepage,63 threads clear 2page,1 thread
>     clear 34 page,so the entire speed is very slow;
> 
>     to solve the first problem,we add a mutex in thread function,and
>     start all threads when all threads finished createion;
>     and the second problem, we spread remainder to other threads,in
>     situation that 160 hugepage and 64 threads, there are 32 threads
>     clear 3 pages,and 32 threads clear 2 pages;
> [test]:
>     320G 84c VM start time can be reduced to 10s
>     680G 84c VM start time can be reduced to 18s
> 
> Signed-off-by: bauerchen <bauerchen@tencent.com>
> Reviewed-by:Pan Rui <ruippan@tencent.com>
> Reviewed-by:Ivan Ren <ivanren@tencent.com>
> ---
>  util/oslib-posix.c | 44 ++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 36 insertions(+), 8 deletions(-)
> 
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index 5a291cc..e97369b 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -76,6 +76,10 @@ static MemsetThread *memset_thread;
>  static int memset_num_threads;
>  static bool memset_thread_failed;
>  
> +static QemuMutex page_mutex;
> +static QemuCond page_cond;
> +static volatile bool thread_create_flag;
> +
>  int qemu_get_thread_id(void)
>  {
>  #if defined(__linux__)
> @@ -403,6 +407,14 @@ static void *do_touch_pages(void *arg)
>      MemsetThread *memset_args = (MemsetThread *)arg;
>      sigset_t set, oldset;
>  
> +    /*wait for all threads create finished */
> +    qemu_mutex_lock(&page_mutex);
> +    while(!thread_create_flag){
> +        qemu_cond_wait(&page_cond, &page_mutex);
> +    }
> +    qemu_mutex_unlock(&page_mutex);
> +
> +
>      /* unblock SIGBUS */
>      sigemptyset(&set);
>      sigaddset(&set, SIGBUS);
> @@ -448,30 +460,46 @@ static inline int get_memset_num_threads(int smp_cpus)
>      return ret;
>  }
>  
> +static void calc_page_per_thread(size_t numpages, int memset_threads, size_t *pages_per_thread){
> +    int avg = numpages / memset_threads + 1;
> +    int i = 0;
> +    int last = avg * memset_threads - numpages;
> +    for (i = 0; i < memset_threads; i++)
> +    {
> +        if(memset_threads - i <= last){
> +            pages_per_thread[i] = avg - 1;
> +        }else
> +            pages_per_thread[i] = avg;
> +    }
> +}
> +
>  static bool touch_all_pages(char *area, size_t hpagesize, size_t numpages,
>                              int smp_cpus)
>  {
> -    size_t numpages_per_thread;
> -    size_t size_per_thread;
> +    size_t *numpages_per_thread;
>      char *addr = area;
>      int i = 0;
>  
>      memset_thread_failed = false;
> +    thread_create_flag = false;
>      memset_num_threads = get_memset_num_threads(smp_cpus);
> +    numpages_per_thread = g_new0(size_t, memset_num_threads);
>      memset_thread = g_new0(MemsetThread, memset_num_threads);
> -    numpages_per_thread = (numpages / memset_num_threads);
> -    size_per_thread = (hpagesize * numpages_per_thread);
> +    calc_page_per_thread(numpages, memset_num_threads, numpages_per_thread);
> +
>      for (i = 0; i < memset_num_threads; i++) {
>          memset_thread[i].addr = addr;
> -        memset_thread[i].numpages = (i == (memset_num_threads - 1)) ?
> -                                    numpages : numpages_per_thread;
> +        memset_thread[i].numpages = numpages_per_thread[i];
>          memset_thread[i].hpagesize = hpagesize;
>          qemu_thread_create(&memset_thread[i].pgthread, "touch_pages",
>                             do_touch_pages, &memset_thread[i],
>                             QEMU_THREAD_JOINABLE);
> -        addr += size_per_thread;
> -        numpages -= numpages_per_thread;
> +        addr += numpages_per_thread[i] * hpagesize;
> +        numpages -= numpages_per_thread[i];
>      }
> +    thread_create_flag = true;
> +    qemu_cond_broadcast(&page_cond);
> +
>      for (i = 0; i < memset_num_threads; i++) {
>          qemu_thread_join(&memset_thread[i].pgthread);
>      }
> -- 
> 1.8.3.1
>