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

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Message-ID: <54AE43E3.60209@suse.cz>
Date:	Thu, 08 Jan 2015 09:46:27 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
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 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.

>> 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.

> Thanks.
> 
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