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

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Message-ID: <20160218095536.GA503@swordfish>
Date:	Thu, 18 Feb 2016 18:55:36 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Joonsoo Kim <js1304@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	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

Hello Joonsoo,

On (02/18/16 17:28), Joonsoo Kim wrote:
> 2016-02-18 12:02 GMT+09:00 Sergey Senozhatsky
> <sergey.senozhatsky.work@...il.com>:
> > ZS_MAX_PAGES_PER_ZSPAGE does not have to be order or 2. The existing
> > limit of 4 pages per zspage sets a tight limit on ->huge classes, which
> > results in increased memory wastage and consumption.
> 
> 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?

MAX_PHYSMEM_BITS	36
_PFN_BITS		36 - 12
OBJ_INDEX_BITS		(32 - (36 - 12) - 1)
ZS_MIN_ALLOC_SIZE	MAX(32, 4 << 12 >> (32 - (36 - 12) - 1))  !=  32

	-ss