From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with ESMTP id 4AB0D6B02A3 for ; Thu, 8 Jul 2010 21:14:11 -0400 (EDT) Received: from kpbe20.cbf.corp.google.com (kpbe20.cbf.corp.google.com [172.25.105.84]) by smtp-out.google.com with ESMTP id o691E7bm008246 for ; Thu, 8 Jul 2010 18:14:07 -0700 Received: from pxi19 (pxi19.prod.google.com [10.243.27.19]) by kpbe20.cbf.corp.google.com with ESMTP id o691DKbt005400 for ; Thu, 8 Jul 2010 18:14:05 -0700 Received: by pxi19 with SMTP id 19so702330pxi.12 for ; Thu, 08 Jul 2010 18:14:05 -0700 (PDT) Date: Thu, 8 Jul 2010 18:13:55 -0700 (PDT) From: Hugh Dickins Subject: Re: [PATCH]shmem: reduce one time of locking in pagefault In-Reply-To: <1278465346.11107.8.camel@sli10-desk.sh.intel.com> Message-ID: References: <1278465346.11107.8.camel@sli10-desk.sh.intel.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Shaohua Li Cc: lkml , linux-mm , Andrew Morton , Andi Kleen , "Zhang, Yanmin" , Tim Chen List-ID: On Wed, 7 Jul 2010, Shaohua Li wrote: > I'm running a shmem pagefault test case (see attached file) under a 64 CPU > system. Profile shows shmem_inode_info->lock is heavily contented and 100% > CPUs time are trying to get the lock. In the pagefault (no swap) case, > shmem_getpage gets the lock twice, the last one is avoidable if we prealloc a > page so we could reduce one time of locking. This is what below patch does. Right. As usual, I'm rather unenthusiastic about a patch which has to duplicate code paths to satisfy an artificial testcase; but I can see the appeal. We can ignore that you're making the swap path slower, that will be lost in its noise. I did like the way the old code checked the max_blocks limit before it let you allocate the page: whereas you might have many threads simultaneously over-allocating before reaching that check; but I guess we can live with that. > > The result of the test case: > 2.6.35-rc3: ~20s > 2.6.35-rc3 + patch: ~12s > so this is 40% improvement. Was that with or without Tim's shmem_sb_info max_blocks scalability changes (that I've still not studied)? Or max_blocks 0 (unlimited)? I notice your test case lets each thread fault in from its own disjoint part of the whole area. Please also test with each thread touching each page in the whole area at the same time: which I think is just as likely a case, but not obvious to me how well it would work with your changes - what numbers does it show? > > One might argue if we could have better locking for shmem. But even shmem is lockless, > the pagefault will soon have pagecache lock heavily contented because shmem must add > new page to pagecache. So before we have better locking for pagecache, improving shmem > locking doesn't have too much improvement. I did a similar pagefault test against > a ramfs file, the test result is ~10.5s. > > Signed-off-by: Shaohua Li > > diff --git a/mm/shmem.c b/mm/shmem.c > index f65f840..c5f2939 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c ... > @@ -1258,7 +1258,19 @@ repeat: > if (error) > goto failed; > radix_tree_preload_end(); > + if (sgp != SGP_READ) { Don't you need to check that prealloc_page is not already set there? There are several places in the swap path where it has to goto repeat. > + /* don't care if this successes */ > + prealloc_page = shmem_alloc_page(gfp, info, idx); > + if (prealloc_page) { > + if (mem_cgroup_cache_charge(prealloc_page, > + current->mm, GFP_KERNEL)) { > + page_cache_release(prealloc_page); > + prealloc_page = NULL; > + } > + } > + } > } Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org