linux-kernel - Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in get_pages_per

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Message-ID: <20160506093342.GB488@swordfish>
Date:	Fri, 6 May 2016 18:33:42 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Ganesh Mahendran <opensource.ganesh@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Nitin Gupta <ngupta@...are.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in
 get_pages_per_zspage()

On (05/06/16 18:08), Sergey Senozhatsky wrote:
[..]
> and it's not 45 iterations that we are getting rid of, but around 31:
> not every class reaches it's ideal 100% ratio on the first iteration.
> so, no, sorry, I don't think the patch really does what we want.


to be clear, what I meant was:

  495 `cmp' + 15 `cmp je'                         IN
  31 `mov cltd idiv mov sub imul cltd idiv cmp'   OUT

IN > OUT.


CORRECTION here:

> * by the way, we don't even need `cltd' in those calculations. the
> reason why gcc puts cltd is because ZS_MAX_PAGES_PER_ZSPAGE has the
> 'wrong' data type. the patch to correct it is below (not a formal
> patch).

no, we need cltd there. but ZS_MAX_PAGES_PER_ZSPAGE also affects
ZS_MIN_ALLOC_SIZE, which is used in several places, like
get_size_class_index(). that's why ZS_MAX_PAGES_PER_ZSPAGE data
type change `improves' zs_malloc().

	-ss