On Fri, Mar 11, 2022 at 1:42 PM Paul Menzel [off-list ref] wrote:

Dear Song,


Am 11.03.22 um 17:59 schrieb Song Liu:

quoted

On Fri, Mar 11, 2022 at 6:16 AM Jens Axboe [off-list ref] wrote:

quoted

On 3/10/22 5:07 PM, Jens Axboe wrote:

quoted

In any case, just doing larger reads would likely help quite a bit, but
would still be nice to get to the bottom of why we're not seeing the
level of merging we expect.

Song, can you try this one? It'll do the dispatch in a somewhat saner
fashion, bundling identical queues. And we'll keep iterating the plug
list for a merge if we have multiple disks, until we've seen a queue
match and checked.

This one works great! We are seeing 99% read request merge and
500kB+ average read size. The original patch in this thread only got
88% and 34kB for these two metrics.

Nice. I am curious, how these metrics can be obtained?

We can use tools as iostat:
iostat -mx 2
Device            r/s     w/s     rMB/s     wMB/s   rrqm/s   wrqm/s
%rrqm  %wrqm r_await w_await aqu-sz rareq-sz wareq-sz  svctm  %util
sdb           3176.50    1.00    100.57      0.00 22503.00     0.00
87.63   0.00   10.22    3.50  32.46    32.42     4.00   0.24  76.60
sdi           3167.00    1.00    100.57      0.00 22512.50     0.00
87.67   0.00   11.58    4.00  36.68    32.52     4.00   0.24  77.55

The two metrics we used here are %rrqm and rareq-sz.

Thanks,
Song