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: > > http://marc.info/?l=linux-raid&m=126944260704907&w=2 > > ...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. Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
