linux-kernel - Re: [RFC PATCH 3/3] mm/zsmalloc: change ZS_MAX_PAGES_PER

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Date:	Fri, 19 Feb 2016 10:19:57 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH 3/3] mm/zsmalloc: change ZS_MAX_PAGES_PER_ZSPAGE

2016-02-18 19:19 GMT+09:00 Sergey Senozhatsky
<sergey.senozhatsky.work@...il.com>:
> On (02/18/16 18:55), Sergey Senozhatsky wrote:
>> > There is a reason that it is order of 2. Increasing ZS_MAX_PAGES_PER_ZSPAGE
>> > is related to ZS_MIN_ALLOC_SIZE. If we don't have enough OBJ_INDEX_BITS,
>> > ZS_MIN_ALLOC_SIZE would be increase and it causes regression on some
>> > system.
>>
>> Thanks!
>>
>> do you mean PHYSMEM_BITS != BITS_PER_LONG systems? PAE/LPAE? isn't it
>> the case that on those systems ZS_MIN_ALLOC_SIZE already bigger than 32?

Indeed.

> I mean, yes, there are ZS_ALIGN requirements that I completely ignored,
> thanks for pointing that out.
>
> just saying, not insisting on anything, theoretically, trading 32 bit size
> objects in exchange of reducing a much bigger memory wastage is sort of
> interesting. zram stores objects bigger than 3072 as huge objects, leaving

I'm also just saying. :)
On the above example system which already uses 128 byte min class,
your change makes it to 160 or 192. It could make a more trouble than
you thought.

> 4096-3072 bytes unused, and it'll take 4096-3072/32 = 4000  32 bit objects
> to beat that single 'bad' compression object in storing inefficiency...

Where does 4096-3072/32 calculation comes from? I'm not familiar to recent
change on zsmalloc such as huge class so can't understand this calculation.

> well, patches 0001/0002 are trying to address this a bit, but the biggest
> problem is still there: we have too many ->huge classes and they are a bit
> far from good.

Agreed. And I agree your patchset, too.

Anyway, could you answer my other questions on original reply?

Thanks.