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

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Date:	Fri, 6 May 2016 12:09:35 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Ganesh Mahendran <opensource.ganesh@...il.com>, ngupta@...are.org,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/zsmalloc: avoid unnecessary iteration in
 get_pages_per_zspage()

On Thu, May 05, 2016 at 07:03:29PM +0900, Sergey Senozhatsky wrote:
> On (05/05/16 13:17), Ganesh Mahendran wrote:
> > if we find a zspage with usage == 100%, there is no need to
> > try other zspages.
> 
> Hello,
> 
> well... we iterate there from 0 to 1<<2, which is not awfully
> a lot to break it in the middle, and we do this only when we
> initialize a new pool (for every size class).
> 
> the check is
>  - true   15 times
>  - false  492 times

Thanks for the data, Sergey!

> 
> so it _sort of_ feels like this new if-condition doesn't
> buy us a lot, and most of the time it just sits there with
> no particular gain. let's hear from Minchan.
> 

I agree with Sergey.
First of al, I appreciates your patch, Ganesh! But as Sergey pointed
out, I don't see why it improves current zsmalloc.
If you want to merge strongly, please convince me with more detail
reason.

Thanks.


> 	-ss
> 
> > Signed-off-by: Ganesh Mahendran <opensource.ganesh@...il.com>
> > Cc: Minchan Kim <minchan@...nel.org>
> > Cc: Nitin Gupta <ngupta@...are.org>
> > Cc: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
> > ---
> >  mm/zsmalloc.c |    3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index fda7177..310c7b0 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -765,6 +765,9 @@ static int get_pages_per_zspage(int class_size)
> >  		if (usedpc > max_usedpc) {
> >  			max_usedpc = usedpc;
> >  			max_usedpc_order = i;
> > +
> > +			if (max_usedpc == 100)
> > +				break;
> >  		}
> >  	}
> >  
> > -- 
> > 1.7.9.5
> >