Hi, 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. Simpliy using several threads doesn't work. conf->device_lock is a global lock which is heavily contended. The first 7 patches in the set are trying to address this problem. With them, when several threads are handling stripe, device_lock is still contended but takes much less cpu time and not the heavist locking any more. Even the 8th patch isn't accepted, the first 7 patches look good to merge. With the locking issue solved (at least largely), switching stripe handling to multiple threads is trival. 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. Comments and suggestions are welcome! 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
