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

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

Hello Minchan,

On (01/29/16 16:28), Minchan Kim wrote:
> Hello Sergey,
> 
> Sorry to late response. Thesedays, I'm really busy with personal
> stuff.

sure, no worries :)

> On Tue, Jan 26, 2016 at 09:03:59PM +0900, Sergey Senozhatsky wrote:
> > I've been asked several very simple questions:
> > a) How can I ensure that zram uses (or used) several compression
> >    streams?
> 
> Why does he want to ensure several compression streams?
> As you know well, zram handle it dynamically.
> 
> If zram cannot allocate more streams, it means the system is
> heavily fragmented or memory pressure at that time so there
> is no worth to add more stream, I think.
> 
> Could you elaborate it more why he want to know it and what
> he expect from that?

good questions. I believe mostly it's about fine-tuning on a
per-device basis, which is getting especially tricky when zram
devices are used as a sort of in-memory tmp storage for various
applications (black boxen).

> > b) What is the current number of comp streams (how much memory
> >    does zram *actually* use for compression streams, if there are
> >    more than one stream)?
> 
> Hmm, in the kernel, there are lots of example subsystem
> we cannot know exact memory usage. Why does the user want
> to know exact memory usage of zram? What is his concern?

certainly true. probably some of those sub-systems/drivers have some
sort of LRU, or shrinker callbacks, to release unneeded memory back.
zram only allocates streams, and it basically hard to tell how many:
up to max_comp_streams, which can be larger than the number of cpus
on the system; because we keep preemption enabled (I didn't realize
that until I played with the patch) around
zcomp_strm_find()/zcomp_strm_release():

	zram_bvec_write()
	{
		...
		zstrm = zcomp_strm_find(zram->comp);
>> can preempt
		user_mem = kmap_atomic(page);
>> now atomic
		zcomp_compress()
		...
		kunmap_atomic()
>> can preempt
		zcomp_strm_release()
		...
	}

so how many streams I can have on my old 4-cpus x86_64 box?

10?
yes.

# cat /sys/block/zram0/mm_stat
630484992  9288707 13103104        0 13103104    16240        0       10

16?
yes.

# cat /sys/block/zram0/mm_stat
1893117952 25296718 31354880        0 31354880    15342        0       16

21?
yes.

# cat /sys/block/zram0/mm_stat
1893167104 28499936 46616576        0 46616576    15330        0       21

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.

10 echo X > /sys/block/zramX/max_comp_streams
20 do tests
30 cat /sys/block/zramX/mm_stat
40 update X (increase/decrease) if needed, otherwise break
50 goto 10


> In advance, sorry for slow response.

no prob, my response wasn't super fast either :)

	-ss