Re: [PATCH] zram: export the number of available comp streams

From: Sergey Senozhatsky
Date: Thu Mar 17 2016 - 21:08:46 EST

Hello Minchan,

On (03/18/16 09:32), Minchan Kim wrote:
> > do I need 21? may be no. do I nede 18? if 18 streams are needed only 10%
> > of the time (I can figure it out by doing repetitive cat zramX/mm_stat),
> > then I can set max_comp_streams to make 90% of applications happy, e.g.
> > max_comp_streams to 10, and save some memory.
> >
> Okay. Let's go back to zcomp design decade. As you remember, the reason
> we separated single and multi stream code was performance caused by
> locking scheme(ie, mutex_lock in single stream model was really fast
> than sleep/wakeup model in multi stream).
> If we could overcome that problem back then, we should have gone to
> multi stream code default.

yes, IIRC I wanted to limit the number of streams by the number of
online CPUs (or was it 2*num_online_cpus()?), and thus change the
number of streams dynamically (because CPUs can go on and off line);
and create at least num_online_cpus() streams during device

the reason for a single-stream zram IIRC were setups in which
zram is used as a swap device. streams require some memory, after
all. and then we discovered that mutex spin on owner boosts single
stream zram significantly.

> How about using *per-cpu* streams?

OK. instead of list of idle streams use per-cpu pointer and process
CPU_FOO notifications. that can work, sounds good to me.

> I remember you wanted to create max number of comp streams statically
> although I didn't want at that time but I change my decision.
> Let's allocate comp stream statically but remove max_comp_streams
> knob. Instead, by default, zram alloctes number of streams according
> to the number of online CPU.

OK. removing `max_comp_streams' will take another 2 years. That's
a major change, we can leave it for longer, just make it nop.

> So I think we can solve locking scheme issue in single stream
> , guarantee parallel level as well as enhancing performance with
> no locking.
> Downside with the approach is that unnecessary memory space reserve
> although zram might be used 1% of running system time. But we
> should give it up for other benefits

aha, ok.

> (ie, simple code, removing
> max_comp_streams knob, no need to this your stat, guarantee parallel
> level, guarantee consumed memory space).

I'll take a look and prepare some numbers (most likely next week).

> What do you think about it?

so should I ask Andrew to drop this patch?