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Date: Thu, 18 Feb 2016 18:55:36 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [RFC PATCH 3/3] mm/zsmalloc: change ZS_MAX_PAGES_PER_ZSPAGE
Message-ID: <20160218095536.GA503@swordfish>
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To: Joonsoo Kim <js1304@gmail.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Minchan Kim <minchan@kernel.org>, Linux Memory Management List <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

Hello Joonsoo,

On (02/18/16 17:28), Joonsoo Kim wrote:
> 2016-02-18 12:02 GMT+09:00 Sergey Senozhatsky
> <sergey.senozhatsky.work@gmail.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

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