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

[Vlastimil Babka <vbabka@suse.cz>]

On 07/19/2016 12:23 AM, Thomas Jarosch wrote:
> 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.

I don't see anything either, might be some change e.g. under fs/ though. 
How about git bisect?

>> 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?

Probably not, but might be possible with live crash session.

> 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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