Re: [Bug 64121] New: [BISECTED] "mm" performance regression updating from 3.2 to 3.3

[Thomas Jarosch <thomas.jarosch@intra2net.com>]

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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