> -----Original Message----- > From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of > Ilya Dryomov > Sent: 11 June 2015 12:33 > To: Nick Fisk > Cc: ceph-users > Subject: Re: krbd splitting large IO's into smaller IO's > > On Thu, Jun 11, 2015 at 2:23 PM, Ilya Dryomov <idryomov@xxxxxxxxx> > wrote: > > On Wed, Jun 10, 2015 at 7:07 PM, Ilya Dryomov <idryomov@xxxxxxxxx> > wrote: > >> On Wed, Jun 10, 2015 at 7:04 PM, Nick Fisk <nick@xxxxxxxxxx> wrote: > >>>> > >> -----Original Message----- > >>>> > >> From: Ilya Dryomov [mailto:idryomov@xxxxxxxxx] > >>>> > >> Sent: 10 June 2015 14:06 > >>>> > >> To: Nick Fisk > >>>> > >> Cc: ceph-users > >>>> > >> Subject: Re: krbd splitting large IO's into > >>>> > >> smaller IO's > >>>> > >> > >>>> > >> On Wed, Jun 10, 2015 at 2:47 PM, Nick Fisk <nick@xxxxxxxxxx> > wrote: > >>>> > >> > Hi, > >>>> > >> > > >>>> > >> > Using Kernel RBD client with Kernel 4.03 (I have also tried > >>>> > >> > some older kernels with the same effect) and IO is being > >>>> > >> > split into smaller IO's which is having a negative impact on > performance. > >>>> > >> > > >>>> > >> > cat /sys/block/sdc/queue/max_hw_sectors_kb > >>>> > >> > 4096 > >>>> > >> > > >>>> > >> > cat /sys/block/rbd0/queue/max_sectors_kb > >>>> > >> > 4096 > >>>> > >> > > >>>> > >> > Using DD > >>>> > >> > dd if=/dev/rbd0 of=/dev/null bs=4M > >>>> > >> > > >>>> > >> > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s > >>>> avgrq-sz > >>>> > >> > avgqu-sz await r_await w_await svctm %util > >>>> > >> > rbd0 0.00 0.00 201.50 0.00 25792.00 0.00 > >>>> 256.00 > >>>> > >> > 1.99 10.15 10.15 0.00 4.96 100.00 > >>>> > >> > > >>>> > >> > > >>>> > >> > Using FIO with 4M blocks > >>>> > >> > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s > >>>> avgrq-sz > >>>> > >> > avgqu-sz await r_await w_await svctm %util > >>>> > >> > rbd0 0.00 0.00 232.00 0.00 118784.00 0.00 > >>>> 1024.00 > >>>> > >> > 11.29 48.58 48.58 0.00 4.31 100.00 > >>>> > >> > > >>>> > >> > Any ideas why IO sizes are limited to 128k (256 blocks) in > >>>> > >> > DD's case and 512k in Fio's case? > >>>> > >> > >>>> > >> 128k vs 512k is probably buffered vs direct IO - add > >>>> > >> iflag=direct to your dd invocation. > >>>> > > > >>>> > > Yes, thanks for this, that was the case > >>>> > > > >>>> > >> > >>>> > >> As for the 512k - I'm pretty sure it's a regression in our > >>>> > >> switch to blk-mq. I tested it around 3.18-3.19 and saw steady > >>>> > >> 4M IOs. I hope we are just missing a knob - I'll take a look. > >>>> > > > >>>> > > I've tested both 4.03 and 3.16 and both seem to be split into 512k. > >>>> > > Let > >>>> me > >>>> > know if you need me to test any other particular version. > >>>> > > >>>> > With 3.16 you are going to need to adjust max_hw_sectors_kb / > >>>> > max_sectors_kb as discussed in Dan's thread. The patch that > >>>> > fixed that in the block layer went into 3.19, blk-mq into 4.0 - try 3.19. > >>>> > >>>> Sorry should have mentioned, I had adjusted both of them on the > >>>> 3.16 kernel to 4096. > >>>> I will try 3.19 and let you know. > >>> > >>> Better with 3.19, but should I not be seeing around 8192, or am I > >>> getting my blocks and bytes mixed up? > >>> > >>> Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz > >>> avgqu-sz await r_await w_await svctm %util > >>> rbd0 72.00 0.00 24.00 0.00 49152.00 0.00 4096.00 > >>> 1.96 82.67 82.67 0.00 41.58 99.80 > >> > >> I'd expect 8192. I'm getting a box for investigation. > > > > OK, so this is bug in the blk-mq part of block layer. There is no > > plugging going on in the single hardware queue (i.e. krbd) case - it > > never once plugs the queue, and that means no request merging is done > > for your direct sequential read test. It gets 512k bios and those > > same 512k requests are issued to krbd. > > > > While queue plugging may not make sense in the multi queue case, I'm > > pretty sure it's supposed to plug in the single queue case. Looks > > like use_plug logic in blk_sq_make_request() is busted. > > It turns out to be a year old regression. Before commit 07068d5b8ed8 > ("blk-mq: split make request handler for multi and single queue") it used to > be (reads are considered sync) > > use_plug = !is_flush_fua && ((q->nr_hw_queues == 1) || !is_sync); > > and now it is > > use_plug = !is_flush_fua && !is_sync; > > in a function that is only called if q->nr_hw_queues == 1. > > This is getting fixed by "blk-mq: fix plugging in blk_sq_make_request" > from Jeff Moyer - http://article.gmane.org/gmane.linux.kernel/1941750. > Looks like it's on its way to mainline along with some other blk-mq plugging > fixes. That's great, do you think it will make 4.2? > > Thanks, > > Ilya > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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