Re: [Xen-devel] IO speed limited by size of IO request (for RBD driver)

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, May 08, 2013 at 11:14:26AM +0000, Felipe Franciosi wrote:
> However we didn't "prove" it properly, I think it is worth mentioning that 
> this boils down to what we originally thought it was:
> Steven's environment is writing to a filesystem in the guest. On top of that, 
> it's using the guest's buffer cache to do the writes.

If he is using O_DIRECT it bypasses the cache in the guest.

> This means that we cannot (easily?) control how the cache and the fs are 
> flushing these writes through blkfront/blkback.
> 
> In other words, it's very likely that it generates a workload that simply 
> doesn't perform well on the "stock" PV protocol.
> This is a good example of how indirect descriptors help (remembering Roger 
> and I were struggling to find use cases where indirect descriptors showed a 
> substantial gain).
> 
> Cheers,
> Felipe
> 
> -----Original Message-----
> From: Roger Pau Monne 
> Sent: 08 May 2013 11:45
> To: Steven Haigh
> Cc: Felipe Franciosi; xen-devel@xxxxxxxxxxxxx
> Subject: Re: IO speed limited by size of IO request (for RBD driver)
> 
> On 08/05/13 12:32, Steven Haigh wrote:
> > On 8/05/2013 6:33 PM, Roger Pau Monné wrote:
> >> On 08/05/13 10:20, Steven Haigh wrote:
> >>> On 30/04/2013 8:07 PM, Felipe Franciosi wrote:
> >>>> I noticed you copied your results from "dd", but I didn't see any 
> >>>> conclusions drawn from experiment.
> >>>>
> >>>> Did I understand it wrong or now you have comparable performance on dom0 
> >>>> and domU when using DIRECT?
> >>>>
> >>>> domU:
> >>>> # dd if=/dev/zero of=output.zero bs=1M count=2048 oflag=direct
> >>>> 2048+0 records in
> >>>> 2048+0 records out
> >>>> 2147483648 bytes (2.1 GB) copied, 25.4705 s, 84.3 MB/s
> >>>>
> >>>> dom0:
> >>>> # dd if=/dev/zero of=output.zero bs=1M count=2048 oflag=direct
> >>>> 2048+0 records in
> >>>> 2048+0 records out
> >>>> 2147483648 bytes (2.1 GB) copied, 24.8914 s, 86.3 MB/s
> >>>>
> >>>>
> >>>> I think that if the performance differs when NOT using DIRECT, the issue 
> >>>> must be related to the way your guest is flushing the cache. This must 
> >>>> be generating a workload that doesn't perform well on Xen's PV protocol.
> >>>
> >>> Just wondering if there is any further input on this... While DIRECT 
> >>> writes are as good as can be expected, NON-DIRECT writes in certain 
> >>> cases (specifically with a mdadm raid in the Dom0) are affected by 
> >>> about a 50% loss in throughput...
> >>>
> >>> The hard part is that this is the default mode of writing!
> >>
> >> As another test with indirect descriptors, could you change 
> >> xen_blkif_max_segments in xen-blkfront.c to 128 (it is 32 by 
> >> default), recompile the DomU kernel and see if that helps?
> > 
> > Ok, here we go.... compiled as 3.8.0-2 with the above change. 3.8.0-2 
> > is running on both the Dom0 and DomU.
> > 
> > # dd if=/dev/zero of=output.zero bs=1M count=2048
> > 2048+0 records in
> > 2048+0 records out
> > 2147483648 bytes (2.1 GB) copied, 22.1703 s, 96.9 MB/s
> > 
> > avg-cpu:  %user   %nice %system %iowait  %steal   %idle
> >             0.34    0.00   17.10    0.00    0.23   82.33
> > 
> > Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s 
> > avgrq-sz avgqu-sz   await  svctm  %util
> > sdd             980.97 11936.47   53.11  429.78     4.00    48.77 
> > 223.81    12.75   26.10   2.11 101.79
> > sdc             872.71 11957.87   45.98  435.67     3.55    49.30 
> > 224.71    13.77   28.43   2.11 101.49
> > sde             949.26 11981.88   51.30  429.33     3.91    48.90 
> > 225.03    21.29   43.91   2.27 109.08
> > sdf             915.52 11968.52   48.58  428.88     3.73    48.92 
> > 225.84    21.44   44.68   2.27 108.56
> > md2               0.00     0.00    0.00 1155.61     0.00    97.51 
> > 172.80     0.00    0.00   0.00   0.00
> > 
> > # dd if=/dev/zero of=output.zero bs=1M count=2048 oflag=direct
> > 2048+0 records in
> > 2048+0 records out
> > 2147483648 bytes (2.1 GB) copied, 25.3708 s, 84.6 MB/s
> > 
> > avg-cpu:  %user   %nice %system %iowait  %steal   %idle
> >             0.11    0.00   13.92    0.00    0.22   85.75
> > 
> > Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s 
> > avgrq-sz avgqu-sz   await  svctm  %util
> > sdd               0.00 13986.08    0.00  263.20     0.00    55.76 
> > 433.87     0.43    1.63   1.07  28.27
> > sdc             202.10 13741.55    6.52  256.57     0.81    54.77 
> > 432.65     0.50    1.88   1.25  32.78
> > sde              47.96 11437.57    1.55  261.77     0.19    45.79 
> > 357.63     0.80    3.02   1.85  48.60
> > sdf            2233.37 11756.13   71.93  191.38     8.99    46.80 
> > 433.90     1.49    5.66   3.27  86.15
> > md2               0.00     0.00    0.00  731.93     0.00    91.49 
> > 256.00     0.00    0.00   0.00   0.00
> > 
> > Now this is pretty much exactly what I would expect the system to do.... 
> > ~96MB/sec buffered, and 85MB/sec direct.
> 
> I'm sorry to be such a PITA, but could you also try with 64? If we have to 
> increase the maximum number of indirect descriptors I would like to set it to 
> the lowest value that provides good performance to prevent using too much 
> memory.
> 
> > So - it turns out that xen_blkif_max_segments at 32 is a killer in the 
> > DomU. Now it makes me wonder what we can do about this in kernels that 
> > don't have your series of patches against it? And also about the 
> > backend stuff in 3.8.x etc?
> 
> There isn't much we can do regarding kernels without indirect descriptors, 
> there's no easy way to increase the number of segments in a request.
> 
> 
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> 

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