Re: [PATCH v4 0/5] kernfs: proposed locking and concurrency improvement

Fox Chen
2021-05-14T02:34:59Z

On Fri, May 14, 2021 at 9:34 AM Ian Kent <raven@themaw.net> 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@themaw.net> 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@gmail.com>
> > > > 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@mail.gmail.com/
>
> 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