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SubjectRe: xfs: does mkfs.xfs require fancy switches to get decent performance? (was Tux3 Report: How fast can we fsync?)


On 05/11/2015 04:17 PM, Theodore Ts'o wrote:
> On Tue, May 12, 2015 at 12:12:23AM +0200, Pavel Machek wrote:
>> Umm, are you sure. If "some areas of disk are faster than others" is
>> still true on todays harddrives, the gaps will decrease the
>> performance (as you'll "use up" the fast areas more quickly).
>
> It's still true. The difference between O.D. and I.D. (outer diameter
> vs inner diameter) LBA's is typically a factor of 2. This is why
> "short-stroking" works as a technique,

That is true, and the effect is not dominant compared to introducing
a lot of extra seeks.

> and another way that people
> doing competitive benchmarking can screw up and produce misleading
> numbers.

If you think we screwed up or produced misleading numbers, could you
please be up front about it instead of making insinuations and
continuing your tirade against benchmarking and those who do it.

> (If you use partitions instead of the whole disk, you have
> to use the same partition in order to make sure you aren't comparing
> apples with oranges.)

You can rest assured I did exactly that.

Somebody complained that things would look much different with seeks
factored out, so here are some new "competitive benchmarks" using
fs_mark on a ram disk:

tasks 1 16 64
------------------------------------
ext4: 231 2154 5439
btrfs: 152 962 2230
xfs: 268 2729 6466
tux3: 315 5529 20301

(Files per second, more is better)

The shell commands are:

fs_mark -dtest -D5 -N100 -L1 -p5 -r5 -s1048576 -w4096 -n1000 -t1
fs_mark -dtest -D5 -N100 -L1 -p5 -r5 -s65536 -w4096 -n1000 -t16
fs_mark -dtest -D5 -N100 -L1 -p5 -r5 -s4096 -w4096 -n1000 -t64

The ram disk removes seek overhead and greatly reduces media transfer
overhead. This does not change things much: it confirms that Tux3 is
significantly faster than the others at synchronous loads. This is
apparently true independently of media type, though to be sure SSD
remains to be tested.

The really interesting result is how much difference there is between
filesystems, even on a ram disk. Is it just CPU or is it synchronization
strategy and lock contention? Does our asynchronous front/back design
actually help a lot, instead of being a disadvantage as you predicted?

It is too bad that fs_mark caps number of tasks at 64, because I am
sure that some embarrassing behavior would emerge at high task counts,
as with my tests on spinning disk.

Anyway, everybody but you loves competitive benchmarks, that is why I
post them. They are not only useful for tracking down performance bugs,
but as you point out, they help us advertise the reasons why Tux3 is
interesting and ought to be merged.

Regards,

Daniel


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