Re: [PATCH v2] mm: swap: prejudgement swap_has_cache to avoid page allocation

[Zhaoyu Liu]

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.