Re: [PATCHSET v3][RFC] Make background writeback not suck

[Jens Axboe <axboe@fb.com>]

On 03/31/2016 06:46 PM, Dave Chinner wrote:
> On Thu, Mar 31, 2016 at 08:29:35AM -0600, Jens Axboe wrote:
>> On 03/31/2016 02:24 AM, Dave Chinner wrote:
>>> On Wed, Mar 30, 2016 at 09:07:48AM -0600, Jens Axboe wrote:
>>>> Hi,
>>>>
>>>> This patchset isn't as much a final solution, as it's demonstration
>>>> of what I believe is a huge issue. Since the dawn of time, our
>>>> background buffered writeback has sucked. When we do background
>>>> buffered writeback, it should have little impact on foreground
>>>> activity. That's the definition of background activity... But for as
>>>> long as I can remember, heavy buffered writers has not behaved like
>>>> that. For instance, if I do something like this:
>>>>
>>>> $ dd if=/dev/zero of=foo bs=1M count=10k
>>>>
>>>> on my laptop, and then try and start chrome, it basically won't start
>>>> before the buffered writeback is done. Or, for server oriented
>>>> workloads, where installation of a big RPM (or similar) adversely
>>>> impacts data base reads or sync writes. When that happens, I get people
>>>> yelling at me.
>>>>
>>>> Last time I posted this, I used flash storage as the example. But
>>>> this works equally well on rotating storage. Let's run a test case
>>>> that writes a lot. This test writes 50 files, each 100M, on XFS on
>>>> a regular hard drive. While this happens, we attempt to read
>>>> another file with fio.
>>>>
>>>> Writers:
>>>>
>>>> $ time (./write-files ; sync)
>>>> real	1m6.304s
>>>> user	0m0.020s
>>>> sys	0m12.210s
>>>
>>> Great. So a basic IO tests looks good - let's through something more
>>> complex at it. Say, a benchmark I've been using for years to stress
>>> the Io subsystem, the filesystem and memory reclaim all at the same
>>> time: a concurent fsmark inode creation test.
>>> (first google hit https://lkml.org/lkml/2013/9/10/46)
>>
>> Is that how you are invoking it as well same arguments?
>
> Yes. And the VM is exactly the same, too - 16p/16GB RAM. Cut down
> version of the script I use:
>
> #!/bin/bash
>
> QUOTA=
> MKFSOPTS=
> NFILES=100000
> DEV=/dev/vdc
> LOGBSIZE=256k
> FSMARK=/home/dave/src/fs_mark-3.3/fs_mark
> MNT=/mnt/scratch
>
> while [ $# -gt 0 ]; do
>          case "$1" in
>          -q)     QUOTA="uquota,gquota,pquota" ;;
>          -N)     NFILES=$2 ; shift ;;
>          -d)     DEV=$2 ; shift ;;
>          -l)     LOGBSIZE=$2; shift ;;
>          --)     shift ; break ;;
>          esac
>          shift
> done
> MKFSOPTS="$MKFSOPTS $*"
>
> echo QUOTA=$QUOTA
> echo MKFSOPTS=$MKFSOPTS
> echo DEV=$DEV
>
> sudo umount $MNT > /dev/null 2>&1
> sudo mkfs.xfs -f $MKFSOPTS $DEV
> sudo mount -o nobarrier,logbsize=$LOGBSIZE,$QUOTA $DEV $MNT
> sudo chmod 777 $MNT
> sudo sh -c "echo 1 > /proc/sys/fs/xfs/stats_clear"
> time $FSMARK  -D  10000  -S0  -n  $NFILES  -s  0  -L  32 \
>          -d  $MNT/0  -d  $MNT/1 \
>          -d  $MNT/2  -d  $MNT/3 \
>          -d  $MNT/4  -d  $MNT/5 \
>          -d  $MNT/6  -d  $MNT/7 \
>          -d  $MNT/8  -d  $MNT/9 \
>          -d  $MNT/10  -d  $MNT/11 \
>          -d  $MNT/12  -d  $MNT/13 \
>          -d  $MNT/14  -d  $MNT/15 \
>          | tee >(stats --trim-outliers | tail -1 1>&2)
> sync
> sudo umount /mnt/scratch

Perfect, thanks!

>>>> The above was run without scsi-mq, and with using the deadline scheduler,
>>>> results with CFQ are similary depressing for this test. So IO scheduling
>>>> is in place for this test, it's not pure blk-mq without scheduling.
>>>
>>> virtio in guest, XFS direct IO -> no-op -> scsi in host.
>>
>> That has write back caching enabled on the guest, correct?
>
> No. It uses virtio,cache=none (that's the "XFS Direct IO" bit above).
> Sorry for not being clear about that.

That's fine, it's one less worry if that's not the case. So if you cat 
the 'write_cache' file in the virtioblk sysfs block queue/ directory, it 
says 'write through'? Just want to confirm that we got that propagated 
correctly.


-- 
Jens Axboe