Hi Johannes,

referring to an old kernel bugzilla issue:
https://bugzilla.kernel.org/show_bug.cgi?id=64121

Am 01.11.2013 um 19:43 wrote Johannes Weiner:

It is a combination of two separate things on these setups.

Traditionally, only lowmem is considered dirtyable so that dirty pages
don't scale with highmem and the kernel doesn't overburden itself with
lowmem pressure from buffers etc.  This is purely about accounting.

My patches on the other hand were about dirty page placement and
avoiding writeback from page reclaim: by subtracting the watermark and
the lowmem reserve (memory not available for user memory / cache) from
each zone's dirtyable memory, we make sure that the zone can always be
rebalanced without writeback.

The problem now is that the lowmem reserves scale with highmem and
there is a point where they entirely overshadow the Normal zone.  This
means that no page cache at all is allowed in lowmem.  Combine this
with how dirtyable memory excludes highmem, and the sum of all
dirtyable memory is nil.  This effectively disables the writeback
cache.

I figure if anything should be fixed it should be the full exclusion
of highmem from dirtyable memory and find a better way to calculate a
minimum.

recently we've updated our production mail server from 3.14.69
to 3.14.73 and it worked fine for a few days. When the box is really
busy (=incoming malware via email), the I/O speed drops to crawl,
write speed is about 5 MB/s on Intel SSDs. Yikes.

The box has 16GB RAM, so it should be a safe HIGHMEM configuration.

Downgrading to 3.14.69 or booting with "mem=15000M" works. I've tested
both approaches and the box was stable. Booting 3.14.73 again triggered
the problem within minutes.

Clearly something with the automatic calculation of the lowmem reserve
crossed a tipping point again, even with the previously considered safe
amount of 16GB RAM for HIGHMEM configs. I don't see anything obvious in
the changelogs from 3.14.69 to 3.14.73, but I might have missed it.

HOWEVER,

the lowmem reserve is highmem/32 per default.  With a Normal zone of
around 900M, this requires 28G+ worth of HighMem to eclipse lowmem
entirely.  This is almost double of what you consider still okay...

is there a way to read out the calculated lowmem reserve via /proc?

It might be interesting the see the lowmem reserve
when booted with mem=15000M or kernel 3.14.69 for comparison.

Do you think it might be worth tinkering with "lowmem_reserve_ratio"?


/proc/meminfo from the box using "mem=15000M" + kernel 3.14.73:

MemTotal:       15001512 kB
HighTotal:      14219160 kB
HighFree:        9468936 kB
LowTotal:         782352 kB
LowFree:          117696 kB
Slab:             430612 kB
SReclaimable:     416752 kB
SUnreclaim:        13860 kB


/proc/meminfo from a similar machine with 16GB RAM + kernel 3.14.73:
(though that machine is just a firewall, so no real disk I/O)

MemTotal:       16407652 kB
HighTotal:      15636376 kB
HighFree:       14415472 kB
LowTotal:         771276 kB
LowFree:          562852 kB
Slab:              34712 kB
SReclaimable:      20888 kB
SUnreclaim:        13824 kB


Any help is appreciated,
Thomas

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