On Fri, May 14, 2021 at 9:34 AM Ian Kent <raven@xxxxxxxxxx> wrote: > > On Thu, 2021-05-13 at 23:37 +0800, Fox Chen wrote: > > Hi Ian > > > > On Thu, May 13, 2021 at 10:10 PM Ian Kent <raven@xxxxxxxxxx> wrote: > > > > > > On Wed, 2021-05-12 at 16:54 +0800, Fox Chen wrote: > > > > On Wed, May 12, 2021 at 4:47 PM Fox Chen <foxhlchen@xxxxxxxxx> > > > > wrote: > > > > > > > > > > Hi, > > > > > > > > > > I ran it on my benchmark ( > > > > > https://github.com/foxhlchen/sysfs_benchmark). > > > > > > > > > > machine: aws c5 (Intel Xeon with 96 logical cores) > > > > > kernel: v5.12 > > > > > benchmark: create 96 threads and bind them to each core then > > > > > run > > > > > open+read+close on a sysfs file simultaneously for 1000 times. > > > > > result: > > > > > Without the patchset, an open+read+close operation takes 550- > > > > > 570 > > > > > us, > > > > > perf shows significant time(>40%) spending on mutex_lock. > > > > > After applying it, it takes 410-440 us for that operation and > > > > > perf > > > > > shows only ~4% time on mutex_lock. > > > > > > > > > > It's weird, I don't see a huge performance boost compared to > > > > > v2, > > > > > even > > > > > > > > I meant I don't see a huge performance boost here and it's way > > > > worse > > > > than v2. > > > > IIRC, for v2 fastest one only takes 40us > > > > > > Thanks Fox, > > > > > > I'll have a look at those reports but this is puzzling. > > > > > > Perhaps the added overhead of the check if an update is > > > needed is taking more than expected and more than just > > > taking the lock and being done with it. Then there's > > > the v2 series ... I'll see if I can dig out your reports > > > on those too. > > > > Apologies, I was mistaken, it's compared to V3, not V2. The previous > > benchmark report is here. > > https://lore.kernel.org/linux-fsdevel/CAC2o3DKNc=sL2n8291Dpiyb0bRHaX=nd33ogvO_LkJqpBj-YmA@xxxxxxxxxxxxxx/ > > Are all these tests using a single file name in the open/read/close > loop? Yes, because It's easy to implement yet enough to trigger the mutex_lock. And you are right It's not a real-life pattern, but on the bright side, it proves there is no original mutex_lock problem anymore. :) > That being the case the per-object inode lock will behave like a > mutex and once contention occurs any speed benefits of a spinlock > over a mutex (or rwsem) will disappear. > > In this case changing from a write lock to a read lock in those > functions and adding the inode mutex will do nothing but add the > overhead of taking the read lock. And similarly adding the update > check function also just adds overhead and, as we see, once > contention starts it has a cumulative effect that's often not > linear. > > The whole idea of a read lock/per-object spin lock was to reduce > the possibility of contention for paths other than the same path > while not impacting same path accesses too much for an overall > gain. Based on this I'm thinking the update check function is > probably not worth keeping, it just adds unnecessary churn and > has a negative impact for same file contention access patterns. > > I think that using multiple paths, at least one per test process > (so if you are running 16 processes use at least 16 different > files, the same in each process), and selecting one at random > for each loop of the open would better simulate real world > access patterns. > > > Ian > thanks, fox
