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From: Minchan Kim <minchan@kernel.org>
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Date: Mon, 9 May 2016 14:01:02 +0900
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Ganesh Mahendran <opensource.ganesh@gmail.com>,
        Minchan Kim <minchan@kernel.org>, Nitin Gupta <ngupta@vflare.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in
 get_pages_per_zspage()
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On Fri, May 06, 2016 at 06:33:42PM +0900, Sergey Senozhatsky wrote:
> 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().

Why not if such simple improves zsmalloc? :)
Please send a patch.

Thanks a lot, Sergey!