Re: Dirty/Writeback fields in /proc/meminfo affected by 20d74bf29c

[Tomas Vondra <tomas@pgaddict.com>]

On 08/04/2016 10:55 PM, Andrew Morton wrote:
> On Mon, 1 Aug 2016 04:36:28 +0200 Tomas Vondra <tomas@pgaddict.com> wrote:
>
>> Hi,
>>
>> While investigating a strange OOM issue on the 3.18.x branch (which
>> turned out to be already fixed by 52c84a95), I've noticed a strange
>> difference in Dirty/Writeback fields in /proc/meminfo depending on
>> kernel version. I'm wondering whether this is expected ...
>>
>> I've bisected the change to 20d74bf29c, added in 3.18.22 (upstream
>> commit 4f258a46):
>>
>>      sd: Fix maximum I/O size for BLOCK_PC requests
>>
>> With /etc/sysctl.conf containing
>>
>>      vm.dirty_background_bytes = 67108864
>>      vm.dirty_bytes = 1073741824
>>
>> a simple "dd" example writing 10GB file
>>
>>      dd if=/dev/zero of=ssd.test.file bs=1M count=10240
>>
>> results in about this on 3.18.21:
>>
>>      Dirty:            740856 kB
>>      Writeback:         12400 kB
>>
>> but on 3.18.22:
>>
>>      Dirty:             49244 kB
>>      Writeback:        656396 kB
>>
>> I.e. it seems to revert the relationship. I haven't identified any
>> performance impact, and apparently for random writes the behavior did
>> not change at all (or at least I haven't managed to reproduce it).
>>
>> But it's unclear to me why setting a maximum I/O size should affect
>> this, and perhaps it has impact that I don't see.
>
> So what appears to be happening here is that background writeback is
> cutting in earlier - the amount of pending writeback ("Dirty") is
> reduced while the amount of active writeback ("Writeback") is
> correspondingly increased.
>
> 4f258a46 had the effect of permitting larger requests into the
> request queue. It's unclear to me why larger requests would cause
> background writeback to cut in earlier - the writeback code doesn't
> even care about individual request sizes, it only cares about
> aggregate pagecache state.
>

Right. Not a kernel expert here, but that's mostly my thinking.

> Less Dirty and more Writeback isn't necessarily a bad thing at all,
> but I don't like mysteries. cc linux-mm to see if anyone else can
> spot-the-difference.
>

I'm not sure if the change has positive or negative impact (or perhaps 
no actual impact), but as a database guy (PostgreSQL) I'm interested in 
this, as the interaction between the database write activity and kernel 
matters to us a lot. So I'm wondering if this change might trigger the 
writeback sooner, etc.

regards
Tomas

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