Re: [patch 8/8] raid5: create multiple threads to handle stripes

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On Tue, Jun 12, 2012 at 09:08:17PM -0700, Dan Williams wrote:
> On Wed, Jun 6, 2012 at 11:45 PM, Shaohua Li <shli@xxxxxxxxxx> wrote:
> > On Thu, Jun 07, 2012 at 11:39:58AM +1000, NeilBrown wrote:
> >> On Mon, 04 Jun 2012 16:02:00 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
> >>
> >> > Like raid 1/10, raid5 uses one thread to handle stripe. In a fast storage, the
> >> > thread becomes a bottleneck. raid5 can offload calculation like checksum to
> >> > async threads. And if storge is fast, scheduling async work and running async
> >> > work will introduce heavy lock contention of workqueue, which makes such
> >> > optimization useless. And calculation isn't the only bottleneck. For example,
> >> > in my test raid5 thread must handle > 450k requests per second. Just doing
> >> > dispatch and completion will make raid5 thread incapable. The only chance to
> >> > scale is using several threads to handle stripe.
> >> >
> >> > With this patch, user can create several extra threads to handle stripe. How
> >> > many threads are better depending on disk number, so the thread number can be
> >> > changed in userspace. By default, the thread number is 0, which means no extra
> >> > thread.
> >> >
> >> > In a 3-disk raid5 setup, 2 extra threads can provide 130% throughput
> >> > improvement (double stripe_cache_size) and the throughput is pretty close to
> >> > theory value. With >=4 disks, the improvement is even bigger, for example, can
> >> > improve 200% for 4-disk setup, but the throughput is far less than theory
> >> > value, which is caused by several factors like request queue lock contention,
> >> > cache issue, latency introduced by how a stripe is handled in different disks.
> >> > Those factors need further investigations.
> >> >
> >> > Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> >>
> >> I think it is great that you have got RAID5 to the point where multiple
> >> threads improve performance.
> >> I really don't like the idea of having to configure that number of threads.
> >>
> >> It would be great if it would auto-configure.
> >> Maybe the main thread could fork aux threads when it notices a high load.
> >> e.g. if it has been servicing requests for more than 100ms without a break,
> >> and the number of threads is less than the number of CPUs, then it forks a new
> >> helper and resets the timer.
> >>
> >> If a thread has been idle for more than 30 minutes, it exits.
> >>
> >> Might that be reasonable?
> >
> > Yep, I bet this patch needs more discussion. auto-configure is preferred. Your
> > idea is worthy doing. However, the concern is if doing auto fork/kill thread,
> > user can't do numa binding, which is important for high speed storage. Maybe
> > have a reasonable default thread number, like one thread one disk? Need more
> > investigations, I'm open to any suggestion in this side.
> The last time I looked at this the btrfs thread pool looked like a
> good candidate:
> ...have not looked if Tejun has made this available as a generic workqueue mode.

I tried to create a UNBOUND workqueue and set max active to the cpu number, so
each cpu will handle one work. In the work, the cpu will handle 8 stripes. The
throughput is relative ok, but CPU utilization is very high compared to just
create 3 or 4 threads like the patch does. There is heavy lock contention in
block queue_lock, since every cpu now dispatches request. There are other
issues like cache, raid5 device_lock has more contention too. It appears too
many threads to handle stripe isn't as good as expected.

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