Re: loop block-mq conversion scalability issues

[Ming Lei <ming.lei@canonical.com>]

Hi Justin,

Thanks for the report.

On Thu, 23 Apr 2015 16:04:10 -0500
"Justin M. Forbes" <jforbes@redhat.com> wrote:

> The block-mq conversion for loop in 4.0 kernels is showing us an
> interesting scalability problem with live CDs (ro, squashfs).  It was
> noticed when testing the Fedora beta that the more CPUs a liveCD image
> was given, the slower it would boot. A 4 core qemu instance or bare
> metal instance took more than twice as long to boot compared to a single
> CPU instance.  After investigating, this came directly to the block-mq
> conversion, reverting these 4 patches will return performance. More
> details are available at
> https://bugzilla.redhat.com/show_bug.cgi?id=1210857
> I don't think that reverting the patches is the ideal solution so I am
> looking for other options.  Since you know this code a bit better than I
> do I thought I would run it by you while I am looking as well.

I can understand the issue because the default @max_active for
alloc_workqueue() is quite big(512), which may cause too much
context switchs, then loop I/O performance gets decreased.

Actually I have written the kernel dio/aio based patch for decreasing
both CPU and memory utilization without sacrificing I/O performance,
and I will try to improve and push the patch during this cycle and hope
it can be merged(kernel/aio.c change is dropped, and only fs change is
needed on fs/direct-io.c).

But the following change should help for your case, could you test it?

---
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index c6b3726..b1cb41d 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1831,7 +1831,7 @@ static int __init loop_init(void)
 	}
 
 	loop_wq = alloc_workqueue("kloopd",
-			WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 0);
+			WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 32);
 	if (!loop_wq) {
 		err = -ENOMEM;
 		goto misc_out;



Thanks,
Ming Lei