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SubjectIncreased Buffers due to patch 56e49d (vmscan: evict use-once pages first), but why exactly?
Hi,
commit 56e49d - "vmscan: evict use-once pages first" changed behavior of
memory management quite a bit which should be fine.
But while tracking down a performance regression I was on the wrong path
for a while suspecting this patch is causing the regression.
Fortunately this was not the case, but I got some interesting data which
I couldn't explain completely and I thought maybe its worth to get it
clarified publicly in case someone else looks at similar data again :-)

All is about the increased amount of "Buffers" accounted as active while
loosing the same portion from "Cache" accounted as inactive in
/proc/meminfo.
I understand that with the patch applied there will be some more
pressure to file pages until the balance of active/inactive file pages
is reached.
But I didn't get how this prefers buffers compared to cache pages (I
assume dropping inactive before active was the case all the time so that
can't be the only difference between buffers/cache).

The scenario I'm running is a low memory system (256M total), that does
sequential I/O with parallel iozone processes.
One process per disk, each process reading a 2Gb file. The issue occurs
independent type of disks I use. File system is ext2.
While bisecting even 4 parallel reads from 2Gb files in /tmp were enough
to see a different amount of buffers in /proc/meminfo.

Looking at the data I got from /proc/meminfo (only significant changes):
before with 56e49d large devs
MemTotal: 250136 kB 250136 kB
MemFree: 6760 kB 6608 kB
Buffers: 2324 kB 34960 kB +32636
Cached: 84296 kB 45860 kB -38436
SwapCached: 392 kB 1416 kB
Active: 6292 kB 38388 kB +32096
Inactive: 89360 kB 51232 kB -38128
Active(anon): 4004 kB 3496 kB
Inactive(anon): 8824 kB 9164 kB
Active(file): 2288 kB 34892 kB +32604
Inactive(file): 80536 kB 42068 kB -38468
Slab: 106624 kB 112364 kB +5740
SReclaimable: 5856 kB 11860 kB +6004
[...]

From slabinfo I know that the slab increase is just secondary due to
more structures to e.g. organize the buffers (buffer_head).
I would understand if file associated memory would now shrink in favor
of non file memory after this patch.
But I can't really see in the code where buffers are favored in
comparison to cached pages - (it very probably makes sense to do so, as
they might contain e.g. the inode data about the files in cache).
I think an explanation how that works might be useful for more people
than just me, so comments welcome.

Kind regards,
Christian

--

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, Open Virtualization

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