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. -- Dan -- 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
