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Date:   Mon, 9 May 2022 18:28:17 -0400
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Byungchul Park <byungchul.park@lge.com>
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Subject: Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)
Message-ID: <YnmVgVQ7usoXnJ1N@mit.edu>
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 <YnmCE2iwa0MSqocr@mit.edu>
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Oh, one other problem with DEPT --- it's SLOW --- the overhead is
enormous.  Using kvm-xfstests[1] running "kvm-xfstests smoke", here
are some sample times:

			LOCKDEP		DEPT
Time to first test	49 seconds	602 seconds
ext4/001      		2 s		22 s
ext4/003		2 s		8 s
ext4/005		0 s		7 s
ext4/020		1 s		8 s
ext4/021		11 s		17 s
ext4/023		0 s		83 s
generic/001		4 s		76 s
generic/002		0 s		11 s
generic/003		10 s		19 s

There are some large variations; in some cases, some xfstests take 10x
as much time or more to run.  In fact, when I first started the
kvm-xfstests run with DEPT, I thought something had hung and that
tests would never start.  (In fact, with gce-xfstests the default
watchdog "something has gone terribly wrong with the kexec" had fired,
and I didn't get any test results using gce-xfstests at all.  If DEPT
goes in without any optimizations, I'm going to have to adjust the
watchdogs timers for gce-xfstests.)

The bottom line is that at the moment, between the false positives,
and the significant overhead imposed by DEPT, I would suggest that if
DEPT ever does go in, that it should be possible to disable DEPT and
only use the existing CONFIG_PROVE_LOCKING version of LOCKDEP, just
because DEPT is S - L - O - W.

[1] https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md

						- Ted

P.S.  Darrick and I both have disabled using LOCKDEP by default
because it slows down ext4 -g auto testing by a factor 2, and xfs -g
auto testing by a factor of 3.  So the fact that DEPT is a factor of
2x to 10x or more slower than LOCKDEP when running various xfstests
tests should be a real concern.