Re: compaction of zspages

[Luigi Semenzato <semenzato@google.com>]

Thank you Seth!

On Wed, Aug 27, 2014 at 3:09 PM, Seth Jennings <sjennings@variantweb.net> wrote:
> On Wed, Aug 27, 2014 at 02:42:52PM -0700, Luigi Semenzato wrote:
>> Hello Minchan and others,
>>
>> I just noticed that the data structures used by zsmalloc have the
>> potential to tie up memory unnecessarily.  I don't call it "leaking"
>> because that memory can be reused, but it's not necessarily returned
>> to the system upon freeing.
>
> Yes, this is a known condition in zsmalloc.
>
> Compaction is not a simple as it seems because zsmalloc returns a handle
> to the user that encodes the pfn.  In order the implement a compaction
> system, there would need to be some notification method to the alert the
> user that their allocation has moved and provide a new handle so the
> user can update its structures.  This is very non-trivial and I'm not
> sure that it can be done safely (i.e.  without races).

Since the handles are opaque, we can add a level of indirection
without affecting users.  Assuming that the overhead is tolerable, or
anyway less than what we're wasting now.  (For some definition of
"less".)

I agree that notification + update would be a huge pain, not really acceptable.

>
> I looked at it a while back and it would be a significant effort.
>
> And yes, if you could do such a thing, you would not want the compaction
> triggered by the shrinkers as the users of zsmalloc are only active
> under memory pressure.  Something like a periodic compaction kthread
> would be the best way (after two minutes of thinking about it).
>
> Seth
>
>
>>
>> I have no idea if this has any impact in practice, but I plan to run a
>> test in the near future.  Also, I am not sure that doing compaction in
>> the shrinkers (as planned according to a comment) is the best
>> approach, because the shrinkers won't be called unless there is
>> considerable pressure, but the compaction would be more effective when
>> there is less pressure.
>>
>> Some more detail here:
>>
>> https://code.google.com/p/chromium/issues/detail?id=408221
>>
>> Should I open a bug on some other tracker?
>>
>> Thank you very much!
>> Luigi
>>
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