Re: [PATCH 1/4] mm/damon/dbgfs: Implement recording feature

From: Andrew Morton <akpm@linux-foundation.org>
To: SeongJae Park <sj@kernel.org>
Cc: Jonathan.Cameron@Huawei.com, amit@kernel.org,
	benh@kernel.crashing.org, corbet@lwn.net, david@redhat.com,
	dwmw@amazon.com, elver@google.com, foersleo@amazon.de,
	gthelen@google.com, markubo@amazon.de, rientjes@google.com,
	shakeelb@google.com, shuah@kernel.org, linux-damon@amazon.com,
	linux-mm@kvack.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/4] mm/damon/dbgfs: Implement recording feature
Date: Sun, 10 Oct 2021 15:01:40 -0700	[thread overview]
Message-ID: <20211010150140.be96f07048079188d9d6b613@linux-foundation.org> (raw)
In-Reply-To: <20211008094509.16179-1-sj@kernel.org>

On Fri,  8 Oct 2021 09:45:06 +0000 SeongJae Park <sj@kernel.org> wrote:

> The user space can get the monitoring results via the 'damon_aggregated'
> tracepoint event.  For simplicity and brevity, the tracepoint events
> have some duplicated information such as 'target_id' and 'nr_regions',
> though.  As a result, its size is greater than really needed.  Also,
> dealing with the tracepoint could be complex for some simple use cases.
> To provide a way for getting more efficient and simple monitoring
> results to user space, this commit implements 'recording' feature in
> 'damon-dbgfs'.
> 
> The feature is exported to the user space via a new debugfs file named
> 'record', which is located in '<debugfs>/damon/' directory.  The file
> allows users to record monitored access patterns in a regular binary
> file in a simple format.

Binary files are troublesome.

Is the format of this file documented anywhere?

I assume that the file's contents will have different representations
depending on host endianness and word size and I further assume that
the provided python script won't handle this very well?

>  The recorded results are first written in an
> in-memory buffer and flushed to a file in batch.  Users can get and set
> the size of the buffer and the path to the result file by reading from
> and writing to the 'record' file.  For example, below commands set the
> buffer to be 4 KiB and the result to be saved in '/damon.data'.

> With a simple test workload[1], recording the tracepoint event using
> 'perf-record' results in 1.7 MiB 'perf.data' file.  When the access
> pattern is recorded via this feature, the size is reduced to 264 KiB. 
> Also, the resulting record file is simple enough to be manipulated by a
> small (100 lines of code) python script which will be introduced by a
> following commit ("selftests/damon: Test recording feature").

How useful and important is this?  I mean, is it tremendously better or
is it a little bit nice to have?  A description of the overall benefit
to DAMON users would be useful in helping others to understand the
benefit of this change.