linux-kernel - Re: [PATCH 1/3] mm/compaction: enhance trace output to know more about compaction internals

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Message-ID: <20150109010441.GA27760@js1304-P5Q-DELUXE>
Date:	Fri, 9 Jan 2015 10:04:42 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/compaction: enhance trace output to know more
 about compaction internals

On Thu, Jan 08, 2015 at 09:46:27AM +0100, Vlastimil Babka wrote:
> On 01/08/2015 09:18 AM, Joonsoo Kim wrote:
> > On Tue, Jan 06, 2015 at 10:05:39AM +0100, Vlastimil Babka wrote:
> >> On 12/03/2014 08:52 AM, Joonsoo Kim wrote:
> >> > It'd be useful to know where the both scanner is start. And, it also be
> >> > useful to know current range where compaction work. It will help to find
> >> > odd behaviour or problem on compaction.
> >> 
> >> Overall it looks good, just two questions:
> >> 1) Why change the pfn output to hexadecimal with different printf layout and
> >> change the variable names and? Is it that better to warrant people having to
> >> potentially modify their scripts parsing the old output?
> > 
> > Deciaml output has really bad readability since we manage all pages by order
> > of 2 which is well represented by hexadecimal. With hex output, we can
> > easily notice whether we move out from one pageblock to another one.
> 
> OK. I don't have any strong objection, maybe Mel should comment on this as the
> author of most of the tracepoints? But if it happens, I think converting the old
> tracepoints to new hexadecimal format should be a separate patch from adding the
> new ones.

Okay.

> 
> >> 2) Would it be useful to also print in the mm_compaction_isolate_template based
> >> tracepoints, pfn of where the particular scanner left off a block prematurely?
> >> It doesn't always match start_pfn + nr_scanned.
> > 
> > With start_pfn and end_pfn, detailed analysis is possible. We can know pageblock
> > where we actually scan and isolate and how much pages we try in that
> > pageblock and can guess why it doesn't become freepage with pageblock
> > order roughly.
> > 
> > nr_scanned is just different metric. end_pfn don't need to match with
> > start_pfn + nr_scanned.
> 
> Well that's part of my point. end_pfn is the end of the pageblock. nr_scanned
> might be lower than end_pfn - start_pfn, because we terminate in the middle of
> the pageblock. But it might be also lower, because we e.g. skip higher-order
> free pages. So we don't recognize where we terminated early.

Ah... now I see your point and found my mistake. My intention is also to
print terminated pfn rather than end_pfn of pageblock. :/
I will change it to print pfn where we terminate scanning.

Thanks.
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