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SubjectRe: [Lse-tech] Re: ext3 performance bottleneck as the number of spindles gets large
Duc Vianney wrote:
> Andrew Morton wrote:
> >If you have time, please test ext2 and/or reiserfs and/or ext3
> >in writeback mode.
> I ran IOzone on ext2fs, ext3fs, JFS, and Reiserfs on an SMP 4-way
> 500MHz, 2.5GB RAM, two 9.1GB SCSI drives. The test partition is 1GB,
> test file size is 128MB, test block size is 4KB, and IO threads varies
> from 1 to 6. When comparing with other file system for this test
> environment, the results on a 2.5.19 SMP kernel show ext3fs is having
> performance problem with Writes and in particularly, with Random Write.
> I think the BKL contention patch would help ext3fs, but I need to verify
> it first.
> The following data are throughput in MB/sec obtained from IOzone
> benchmark running on all file systems installed with default options.
> Kernels 2519smp4 2519smp4 2519smp4 2519smp4
> No of threads=1 ext2-1t jfs-1t ext3-1t reiserfs-1t
> Initial write 138010 111023 29808 48170
> Rewrite 205736 204538 119543 142765
> Read 236500 237235 231860 236959
> Re-read 242927 243577 240284 242776
> Random read 204292 206010 201664 207219
> Random write 180144 180461 1090 121676

ext3 only allows dirty data to remain in memory for five seconds,
whereas the other filesystems allow it for thirty. This is
a reasonable thing to do, but it hurts badly in benchmarks.

If you run a benchmark which takes ext2 ten seconds to
complete, ext2 will do it all in-RAM. But after five
seconds, ext3 will go to disk and the test takes vastly longer.
I suspect that is what is happening here - we're seeing the
difference between disk bandwidth and memory bandwidth.

If you choose a larger file, a shorter file or a longer-running
test then the difference will not be so gross.

You can confirm this by trying a one-gigabyte file instead.

The "Initial write" is fishy. I wonder if the same thing
is happening here - there may have been lots of dirty memory
left in-core (and unaccounted for) after the test completed.
iozone has a `-e' option which causes it to include the fsync()
time in the timing calculations. Using that would give a
better comparison, unless you are specifically trying to test
in-memory performance. And we're not doing that here.

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