Re: zram, OOM, and speed of allocation

[Sonny Rao <sonnyrao@google.com>]

On Thu, Nov 29, 2012 at 11:31 AM, Luigi Semenzato <semenzato@google.com> wrote:
> Oh well, I found the problem, it's laptop_mode.  We keep it on by
> default.  When I turn it off, I can allocate as fast as I can, and no
> OOMs happen until swap is exhausted.
>
> I don't think this is a desirable behavior even for laptop_mode, so if
> anybody wants to help me debug it (or wants my help in debugging it)
> do let me know.
>

Luigi, I thought we disabled Laptop mode a few weeks ago -- due to
undesirable behavior with respect to too many writes happening.
Are you sure it's on?

> Thanks!
> Luigi
>
> On Thu, Nov 29, 2012 at 10:46 AM, Luigi Semenzato <semenzato@google.com> wrote:
>> Minchan:
>>
>> I tried your suggestion to move the call to wake_all_kswapd from after
>> "restart:" to after "rebalance:".  The behavior is still similar, but
>> slightly improved.  Here's what I see.
>>
>> Allocating as fast as I can: 1.5 GB of the 3 GB of zram swap are used,
>> then OOM kills happen, and the system ends up with 1 GB swap used, 2
>> unused.
>>
>> Allocating 10 MB/s: some kills happen when only 1 to 1.5 GB are used,
>> and continue happening while swap fills up.  Eventually swap fills up
>> completely.  This is better than before (could not go past about 1 GB
>> of swap used), but there are too many kills too early.  I would like
>> to see no OOM kills until swap is full or almost full.
>>
>> Allocating 20 MB/s: almost as good as with 10 MB/s, but more kills
>> happen earlier, and not all swap space is used (400 MB free at the
>> end).
>>
>> This is with 200 processes using 20 MB each, and 2:1 compression ratio.
>>
>> So it looks like kswapd is still not aggressive enough in pushing
>> pages out.  What's the best way of changing that?  Play around with
>> the watermarks?
>>
>> Incidentally, I also tried removing the min_filelist_kbytes hacky
>> patch, but, as usual, the system thrashes so badly that it's
>> impossible to complete any experiment.  I set it to a lower minimum
>> amount of free file pages, 10 MB instead of the 50 MB which we use
>> normally, and I could run with some thrashing, but I got the same
>> results.
>>
>> Thanks!
>> Luigi
>>
>>
>> On Wed, Nov 28, 2012 at 4:31 PM, Luigi Semenzato <semenzato@google.com> wrote:
>>> I am beginning to understand why zram appears to work fine on our x86
>>> systems but not on our ARM systems.  The bottom line is that swapping
>>> doesn't work as I would expect when allocation is "too fast".
>>>
>>> In one of my tests, opening 50 tabs simultaneously in a Chrome browser
>>> on devices with 2 GB of RAM and a zram-disk of 3 GB (uncompressed), I
>>> was observing that on the x86 device all of the zram swap space was
>>> used before OOM kills happened, but on the ARM device I would see OOM
>>> kills when only about 1 GB (out of 3) was swapped out.
>>>
>>> I wrote a simple program to understand this behavior.  The program
>>> (called "hog") allocates memory and fills it with a mix of
>>> incompressible data (from /dev/urandom) and highly compressible data
>>> (1's, just to avoid zero pages) in a given ratio.  The memory is never
>>> touched again.
>>>
>>> It turns out that if I don't limit the allocation speed, I see
>>> premature OOM kills also on the x86 device.  If I limit the allocation
>>> to 10 MB/s, the premature OOM kills stop happening on the x86 device,
>>> but still happen on the ARM device.  If I further limit the allocation
>>> speed to 5 Mb/s, the premature OOM kills disappear also from the ARM
>>> device.
>>>
>>> I have noticed a few time constants in the MM whose value is not well
>>> explained, and I am wondering if the code is tuned for some ideal
>>> system that doesn't behave like ours (considering, for instance, that
>>> zram is much faster than swapping to a disk device, but it also uses
>>> more CPU).  If this is plausible, I am wondering if anybody has
>>> suggestions for changes that I could try out to obtain a better
>>> behavior with a higher allocation speed.
>>>
>>> Thanks!
>>> Luigi

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