On Wed, Aug 08 2018, J. Bruce Fields wrote: > On Wed, Aug 08, 2018 at 12:47:22PM -0400, Jeff Layton wrote: >> On Wed, 2018-08-08 at 11:51 +1000, NeilBrown wrote: >> > If you have a many-core machine, and have many threads all wanting to >> > briefly lock a give file (udev is known to do this), you can get quite >> > poor performance. >> > >> > When one thread releases a lock, it wakes up all other threads that >> > are waiting (classic thundering-herd) - one will get the lock and the >> > others go to sleep. >> > When you have few cores, this is not very noticeable: by the time the >> > 4th or 5th thread gets enough CPU time to try to claim the lock, the >> > earlier threads have claimed it, done what was needed, and released. >> > With 50+ cores, the contention can easily be measured. >> > >> > This patchset creates a tree of pending lock request in which siblings >> > don't conflict and each lock request does conflict with its parent. >> > When a lock is released, only requests which don't conflict with each >> > other a woken. >> > >> > Testing shows that lock-acquisitions-per-second is now fairly stable even >> > as number of contending process goes to 1000. Without this patch, >> > locks-per-second drops off steeply after a few 10s of processes. >> > >> > There is a small cost to this extra complexity. >> > At 20 processes running a particular test on 72 cores, the lock >> > acquisitions per second drops from 1.8 million to 1.4 million with >> > this patch. For 100 processes, this patch still provides 1.4 million >> > while without this patch there are about 700,000. >> > >> > NeilBrown >> > >> > --- >> > >> > NeilBrown (4): >> > fs/locks: rename some lists and pointers. >> > fs/locks: allow a lock request to block other requests. >> > fs/locks: change all *_conflict() functions to return bool. >> > fs/locks: create a tree of dependent requests. >> > >> > >> > fs/cifs/file.c | 2 - >> > fs/locks.c | 142 +++++++++++++++++++++++++-------------- >> > include/linux/fs.h | 5 + >> > include/trace/events/filelock.h | 16 ++-- >> > 4 files changed, 103 insertions(+), 62 deletions(-) >> > >> >> Nice work! I looked over this and I think it looks good. >> >> I made an attempt to fix this issue several years ago, but my method >> sucked as it ended up penalizing the unlocking task too much. This is >> much cleaner and should scale well overall, I think. > > I think I also took a crack at this at one point while I was at UM/CITI > and never got anything I was happy with. Looks like good work! > > I remember one main obstacle that I felt like I never had a good > benchmark.... > > How did you choose this workload and hardware? Was it in fact udev > (booting a large machine?), or was there some other motivation? I'm hoping Martin will chime in here - her identified the problem and did most of the testing... NeilBrown > > Not that I'm likely to do it any time soon, but could you share > sufficient details for someone else to reproduce your results? > > --b.
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