Date:   Tue, 5 Feb 2019 08:38:10 -0500
From:   Tom Talpey <tom@...pey.com>
To:     John Hubbard <jhubbard@...dia.com>, john.hubbard@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christian Benvenuti <benve@...co.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerome Glisse <jglisse@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/6] RFC v2: mm: gup/dma tracking

On 2/5/2019 3:22 AM, John Hubbard wrote:
> On 2/4/19 5:41 PM, Tom Talpey wrote:
>> On 2/4/2019 12:21 AM, john.hubbard@...il.com wrote:
>>> From: John Hubbard <jhubbard@...dia.com>
>>>
>>>
>>> Performance: here is an fio run on an NVMe drive, using this for the fio
>>> configuration file:
>>>
>>>      [reader]
>>>      direct=1
>>>      ioengine=libaio
>>>      blocksize=4096
>>>      size=1g
>>>      numjobs=1
>>>      rw=read
>>>      iodepth=64
>>>
>>> reader: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 
>>> 4096B-4096B, ioengine=libaio, iodepth=64
>>> fio-3.3
>>> Starting 1 process
>>> Jobs: 1 (f=1)
>>> reader: (groupid=0, jobs=1): err= 0: pid=7011: Sun Feb  3 20:36:51 2019
>>>     read: IOPS=190k, BW=741MiB/s (778MB/s)(1024MiB/1381msec)
>>>      slat (nsec): min=2716, max=57255, avg=4048.14, stdev=1084.10
>>>      clat (usec): min=20, max=12485, avg=332.63, stdev=191.77
>>>       lat (usec): min=22, max=12498, avg=336.72, stdev=192.07
>>>      clat percentiles (usec):
>>>       |  1.00th=[  322],  5.00th=[  322], 10.00th=[  322], 20.00th=[ 
>>> 326],
>>>       | 30.00th=[  326], 40.00th=[  326], 50.00th=[  326], 60.00th=[ 
>>> 326],
>>>       | 70.00th=[  326], 80.00th=[  330], 90.00th=[  330], 95.00th=[ 
>>> 330],
>>>       | 99.00th=[  478], 99.50th=[  717], 99.90th=[ 1074], 99.95th=[ 
>>> 1090],
>>>       | 99.99th=[12256]
>>
>> These latencies are concerning. The best results we saw at the end of
>> November (previous approach) were MUCH flatter. These really start
>> spiking at three 9's, and are sky-high at four 9's. The "stdev" values
>> for clat and lat are about 10 times the previous. There's some kind
>> of serious queuing contention here, that wasn't there in November.
> 
> Hi Tom,
> 
> I think this latency problem is also there in the baseline kernel, but...
> 
>>
>>>     bw (  KiB/s): min=730152, max=776512, per=99.22%, avg=753332.00, 
>>> stdev=32781.47, samples=2
>>>     iops        : min=182538, max=194128, avg=188333.00, 
>>> stdev=8195.37, samples=2
>>>    lat (usec)   : 50=0.01%, 100=0.01%, 250=0.07%, 500=99.26%, 750=0.38%
>>>    lat (usec)   : 1000=0.02%
>>>    lat (msec)   : 2=0.24%, 20=0.02%
>>>    cpu          : usr=15.07%, sys=84.13%, ctx=10, majf=0, minf=74
>>
>> System CPU 84% is roughly double the November results of 45%. Ouch.
> 
> That's my fault. First of all, I had a few extra, supposedly minor debug
> settings in the .config, which I'm removing now--I'm doing a proper run
> with the original .config file from November, below. Second, I'm not
> sure I controlled the run carefully enough.
> 
>>
>> Did you re-run the baseline on the new unpatched base kernel and can
>> we see the before/after?
> 
> Doing that now, I see:
> 
> -- No significant perf difference between before and after, but
> -- Still high clat in the 99.99th
> 
> =======================================================================
> Before: using commit 8834f5600cf3 ("Linux 5.0-rc5")
> ===================================================
> reader: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 
> 4096B-4096B, ioengine=libaio, iodepth=64
> fio-3.3
> Starting 1 process
> Jobs: 1 (f=1)
> reader: (groupid=0, jobs=1): err= 0: pid=1829: Tue Feb  5 00:08:08 2019
>     read: IOPS=193k, BW=753MiB/s (790MB/s)(1024MiB/1359msec)
>      slat (nsec): min=1269, max=40309, avg=1493.66, stdev=534.83
>      clat (usec): min=127, max=12249, avg=329.83, stdev=184.92
>       lat (usec): min=129, max=12256, avg=331.35, stdev=185.06
>      clat percentiles (usec):
>       |  1.00th=[  326],  5.00th=[  326], 10.00th=[  326], 20.00th=[  326],
>       | 30.00th=[  326], 40.00th=[  326], 50.00th=[  326], 60.00th=[  326],
>       | 70.00th=[  326], 80.00th=[  326], 90.00th=[  326], 95.00th=[  326],
>       | 99.00th=[  347], 99.50th=[  519], 99.90th=[  529], 99.95th=[  537],
>       | 99.99th=[12125]
>     bw (  KiB/s): min=755032, max=781472, per=99.57%, avg=768252.00, 
> stdev=18695.90, samples=2
>     iops        : min=188758, max=195368, avg=192063.00, stdev=4673.98, 
> samples=2
>    lat (usec)   : 250=0.08%, 500=99.18%, 750=0.72%
>    lat (msec)   : 20=0.02%
>    cpu          : usr=12.30%, sys=46.83%, ctx=253554, majf=0, minf=74
>    IO depths    : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, 
>  >=64=100.0%
>       submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, 
>  >=64=0.0%
>       complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.1%, 
>  >=64=0.0%
>       issued rwts: total=262144,0,0,0 short=0,0,0,0 dropped=0,0,0,0
>       latency   : target=0, window=0, percentile=100.00%, depth=64
> 
> Run status group 0 (all jobs):
>     READ: bw=753MiB/s (790MB/s), 753MiB/s-753MiB/s (790MB/s-790MB/s), 
> io=1024MiB (1074MB), run=1359-1359msec
> 
> Disk stats (read/write):
>    nvme0n1: ios=221246/0, merge=0/0, ticks=71556/0, in_queue=704, 
> util=91.35%
> 
> =======================================================================
> After:
> =======================================================================
> reader: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 
> 4096B-4096B, ioengine=libaio, iodepth=64
> fio-3.3
> Starting 1 process
> Jobs: 1 (f=1)
> reader: (groupid=0, jobs=1): err= 0: pid=1803: Mon Feb  4 23:58:07 2019
>     read: IOPS=193k, BW=753MiB/s (790MB/s)(1024MiB/1359msec)
>      slat (nsec): min=1276, max=41900, avg=1505.36, stdev=565.26
>      clat (usec): min=177, max=12186, avg=329.88, stdev=184.03
>       lat (usec): min=178, max=12192, avg=331.42, stdev=184.16
>      clat percentiles (usec):
>       |  1.00th=[  326],  5.00th=[  326], 10.00th=[  326], 20.00th=[  326],
>       | 30.00th=[  326], 40.00th=[  326], 50.00th=[  326], 60.00th=[  326],
>       | 70.00th=[  326], 80.00th=[  326], 90.00th=[  326], 95.00th=[  326],
>       | 99.00th=[  359], 99.50th=[  498], 99.90th=[  537], 99.95th=[  627],
>       | 99.99th=[12125]
>     bw (  KiB/s): min=754656, max=781504, per=99.55%, avg=768080.00, 
> stdev=18984.40, samples=2
>     iops        : min=188664, max=195378, avg=192021.00, stdev=4747.51, 
> samples=2
>    lat (usec)   : 250=0.12%, 500=99.40%, 750=0.46%
>    lat (msec)   : 20=0.02%
>    cpu          : usr=12.44%, sys=47.05%, ctx=252127, majf=0, minf=73
>    IO depths    : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, 
>  >=64=100.0%
>       submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, 
>  >=64=0.0%
>       complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.1%, 
>  >=64=0.0%
>       issued rwts: total=262144,0,0,0 short=0,0,0,0 dropped=0,0,0,0
>       latency   : target=0, window=0, percentile=100.00%, depth=64
> 
> Run status group 0 (all jobs):
>     READ: bw=753MiB/s (790MB/s), 753MiB/s-753MiB/s (790MB/s-790MB/s), 
> io=1024MiB (1074MB), run=1359-1359msec
> 
> Disk stats (read/write):
>    nvme0n1: ios=221203/0, merge=0/0, ticks=71291/0, in_queue=704, 
> util=91.19%
> 
> How's this look to you?

Ok, I'm satisfied the four-9's latency spike is in not your code. :-)
Results look good relative to baseline. Thanks for doublechecking!

Tom.

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