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.
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