[patch 0/8] raid5: improve write performance for fast storage

[Shaohua Li <shli@kernel.org>]

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