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

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On 08/04/2016 10:55 PM, Andrew Morton wrote:
On Mon, 1 Aug 2016 04:36:28 +0200 Tomas Vondra <tomas@xxxxxxxxxxxx> 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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