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Subject[Demo program]: Poor elevator performance in 2.4.0-test9pre6
Ive written a small program to demonstrate the performance problems Ive
been seeing in recent Linux kernels.

The benchmark is a single process which writes and read 8k blocks
round-robin from a number of files.
It is written as a single process so the ordering of the operations is
known and perfectly interleaved. I include the source at the end of this
message.
The files are initially created using sequential writes so that the
files are laid out nicely on disk.

With kernel version 2.4.0-test9pre6 the results are as follows.
The test machine has 128 Megs of memory. The tests accesses 240 Megs of
files so that it can't fit in cache.

If I run it with 8 files of size 30 Megs:

[robert@test25 src]$ ./elv_test 8 30
files created, 240 megs written at 8.96 megs/sec
finished writing 240 megs written at 1.05 megs per sec <<<<<<
finished reading, 240 megs read at 5.848833 megs/sec

If I do the same with a single file of size 240 Megs

[robert@test25 src]$ ./elv_test 1 240
files created, 240 megs written at 11.12 megs/sec
finished writing 240 megs written at 11.08 megs per sec
finished reading, 240 megs read at 12.580521 megs/sec

Comparing this to a similar tiotest run

[robert@test25 src]$ tiotest -f 30 -b 8192 -t 8 -r 0
Tiotest results for 8 concurrent io threads:

,----------------------------------------------------------------------.
| Item | Time | Rate | Usr CPU | Sys CPU |
+-----------------------+----------+--------------+----------+---------+
| Write 240 MBs | 25.5 s | 9.410 MB/s | 0.1 % | 10.0 % |
| Read 240 MBs | 20.4 s | 11.755 MB/s | 0.0 % | 8.8 % |
`----------------------------------------------------------------------'


As the tests demonstrate, we get terrible write performance when a
single processes is writing round robin to a number of files.
There are two possible explanations for this, the single threaded nature
of the program is slowing things down. Or the fact that the files are
being written round robin is slowing us down.

Since I see exactly the same kind of behaviour with the netatalk
benchmark I have been using and the netatalk benchmark isnt single
threaded, I believe that its the round robin interleaving of writes that
leading to the performance problems.

As a comparison, heres the results of the test program in kernel version
2.4.0-test1-ac22.

[robert@testmac25 src]$ ./elv_test 8 30
files created, 240 megs written at 8.24 megs/sec
finished writing 240 megs written at 8.99 megs per sec
finished reading, 240 megs read at 5.849072 megs/sec


Here the write performance is fine. This definitely indicates that its
not the single threaded benchmark thats slowing things down.

As I understand it, the elevator should be dealing with the interleaved
nature of the writes. This seems to be working ok for reads, but it
doesnt seem to be working properly for writes.

The source can be found at http://tltsu.anu.edu.au/~robert/elv_test.c

--
Robert Cohen
TLTSU, Unix support
Australian National University
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