On Wed, Jul 07, 2010 at 09:32:54AM +0800, Andrew Morton wrote: > On Wed, 07 Jul 2010 09:15:46 +0800 Shaohua Li <shaohua.li@xxxxxxxxx> 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. > > I seem to remember complaining about that in 2002 ;) Faulting in a > mapping of /dev/zero is just awful on a 4-way(!). > > > 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. > > > > The result of the test case: > > 2.6.35-rc3: ~20s > > 2.6.35-rc3 + patch: ~12s > > so this is 40% improvement. > > > > 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 <shaohua.li@xxxxxxxxx> > > > > diff --git a/mm/shmem.c b/mm/shmem.c > > index f65f840..c5f2939 100644 > > --- a/mm/shmem.c > > +++ b/mm/shmem.c > > The patch doesn't make shmem_getpage() any clearer :( > > shmem_inode_info.lock appears to be held too much. Surely > lookup_swap_cache() didn't need it (for example). > > What data does shmem_inode_info.lock actually protect? As far as my understanding, it protects shmem swp_entry, which is most used to support swap. It also protects some accounting. If no swap, the lock almost can be removed like tiny-shmem. Thanks, Shaohua -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>