Re: [Jfs-discussion] benchmark results

tytso
2009-12-25T16:11:08Z

On Fri, Dec 25, 2009 at 02:46:31AM +0300, Evgeniy Polyakov wrote:
> > [1] http://samba.org/ftp/tridge/dbench/README
>
> Was not able to resist to write a small notice, what no matter what, but
> whatever benchmark is running, it _does_ show system behaviour in one
> or another condition. And when system behaves rather badly, it is quite
> a common comment, that benchmark was useless. But it did show that
> system has a problem, even if rarely triggered one :)

If people are using benchmarks to improve file system, and a benchmark
shows a problem, then trying to remedy the performance issue is a good
thing to do, of course. Sometimes, though the case which is
demonstrated by a poor benchmark is an extremely rare corner case that
doesn't accurately reflect common real-life workloads --- and if
addressing it results in a tradeoff which degrades much more common
real-life situations, then that would be a bad thing.

In situations where benchmarks are used competitively, it's rare that
it's actually a *problem*. Instead it's much more common that a
developer is trying to prove that their file system is *better* to
gullible users who think that a single one-dimentional number is
enough for them to chose file system X over file system Y.

For example, if I wanted to play that game and tell people that ext4
is better, I'd might pick this graph:

http://btrfs.boxacle.net/repository/single-disk/2.6.29-rc2/2.6.29-rc2/2.6.29-rc2_Mail_server_simulation._num_threads=32.html

On the other hand, this one shows ext4 as the worst compared to all
other file systems:

http://btrfs.boxacle.net/repository/single-disk/2.6.29-rc2/2.6.29-rc2/2.6.29-rc2_Large_file_random_writes_odirect._num_threads=8.html

Benchmarking, like statistics, can be extremely deceptive, and if
people do things like carefully order a tar file so the files are
optimal for a file system, it's fair to ask whether that's a common
thing for people to be doing (either unpacking tarballs or unpacking
tarballs whose files have been carefully ordered for a particular file
systems). When it's the only number used by a file system developer
when trying to convince users they should use their file system, at
least in my humble opinion it becomes murderously dishonest.

- Ted