On Mon, Apr 08, 2024 at 01:27:04PM -0700, Andrew Morton wrote: > On Mon, 8 Apr 2024 20:14:39 +0800 Zhaoyu Liu <liuzhaoyu.zackary@xxxxxxxxxxxxx> wrote: > > > Based on qemu arm64 - latest kernel + 100M memory + 1024M swapfile. > > Create 1G anon mmap and set it to shared, and has two processes > > randomly access the shared memory. When they are racing on swap cache, > > on average, each "alloc_pages_mpol + swapcache_prepare + folio_put" > > took about 1475 us. > > And what effect does this patch have upon the measured time? ANd upon > overall runtime? Hi Andrew, When share memory between two or more processes has swapped and pagefault now, it would readahead swap and call __read_swap_cache_async(). If one of the processes calls swapcache_prepare() and finds that the cache has been EXIST(another process added), it will folio_put on the basis of the alloc_pages_mpol() that has been called, and then try filemap_get_folio() again. I think the page alloc in this process is wasteful. when the memory pressure is large, alloc_pages_mpol() will be time-consuming, so the purpose of my patch is to judge whether the page has cache before page alloc, then skip page alloc and retry filemap_get_folio() to save the time of the function. Thank you. > > > So skip page allocation if SWAP_HAS_CACHE was set, just > > schedule_timeout_uninterruptible and continue to acquire page > > via filemap_get_folio() from swap cache, to speedup > > __read_swap_cache_async.
