Re: [PATCH v2 3/4] zram: zram memory size limitation

[Marc Dietrich]

Am Mittwoch, 20. August 2014, 08:32:25 schrieb Minchan Kim:
> Hello,
> 
> On Tue, Aug 19, 2014 at 10:06:22AM +0200, Marc Dietrich wrote:
> > Am Dienstag, 19. August 2014, 16:54:46 schrieb Minchan Kim:
> > > Since zram has no control feature to limit memory usage,
> > > it makes hard to manage system memrory.
> > > 
> > > This patch adds new knob "mem_limit" via sysfs to set up the
> > > a limit so that zram could fail allocation once it reaches
> > > the limit.
> > 
> > Sorry to jump in late with a probably silly question, but I couldn't find
> > the answer easily. What's the difference between disksize and mem_limit?
> No need to say sorry.
> It was totally my fault because zram documentation sucks.
> 
> The disksize means the size point of view upper layer from block subsystem
> so filesystem based on zram or blockdevice itself(ie, zram0) seen by admin
> will have the disksize size but keep in mind that it's virtual size,
> not compressed. As you know already, zram is backed on volatile storage
> (ie, DRAM) with *compressed form*, not permanent storage.
> 
> The point of this patchset is that anybody cannot expect exact memory
> usage of zram in advance. Acutally, zram folks have estimated it by several
> experiment and assuming zram compression ratio(ex, 2:1 or 3:1) before
> releasing product. But thesedays, embedded platforms have varios workloads
> which cannot be expected when the product was released so compression
> ratio expectation could be wrong sometime so zram could consume lots of
> memory than expected once compression ratio is low.
> 
> It makes admin trouble to manage memeory on the product because there
> is no way to release memory zram is using so that one of the way is
> to limit memory usage of zram from the beginning.
> 
> > I assume the former is uncompressed size (virtual size) and the latter is
> > compressed size (real memory usage)? Maybe the difference should be made
> 
> Right.
> 
> > clearer in the documentation.
> 
> Okay.
> 
> > If disksize is the uncompressed size, why would we want to set this at
> > all?
> 
> For example, we have 500M disksize of zram0 because we assumed 2:1
> compression ratio so that we could guess zram will consume 250M physical
> memory in the end. But our guessing could be wrong so if real compression
> ratio is 4:1, we use up 125M phsyical memory to store 500M uncompressed
> pages. It's good but admin want to use up more memory for zram because we
> saved 100% than expected zram memory but we couldn't becuase upper layer
> point of view from zram, zram is already full by 500M and if zram is used
> for swap, we will encounter OOM. :(
> 
> So, it would be better to increase disksize to 1000M but in this case,
> if compression ratio becomes 4:1 by something(ex, workload change),
> zram can consume 500M physical memory, which is above we expected
> and admin don't want zram to use up system memory too much.
> 
> In summary, we couldn't control exact zram memory usage with only disksize
> by compression ratio.

thanks for your detailed explanation. It's a bit confusing that you can 
specify two limits (for two different layers). I guess a floating disksize is 
not possible, because you wouldn't be able to create a filesystem/swapfile on 
it, so you need to make a *fixed* assumption.

Regards,

Marc

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