This is just to report that I/O performance in the pre kernels is very
bad. System is so sluggish that you don't need any benchmarks to
quantify it, but I did some anyway. :)
This is bonnie output on pre9-3 virgin:
-------Sequential Output-------- ---Sequential Input-- --Random--
-Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
pre9-3 400 15032 64.4 12843 13.8 6202 7.1 11421 38.1 13005 11.6 118.1 1.2
^^^^^ ^^^^^
It looks like a slightly slower/older 5400 disk, is it?
But in fact it is a quite fast (and expensive) 7200rpm disk, which is
capable of this:
-------Sequential Output-------- ---Sequential Input-- --Random--
-Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
9-3-nswap 400 21222 89.7 22244 19.7 7343 8.0 17863 57.3 21030 18.0 118.8 1.1
^^^^^ ^^^^^
This second numbers are generated on the same kernel version, but with
disabled swap_out() function.
Memory balancing is killing I/O. It is very common to see system
swapping loads of pages in/out with only one I/O intensive process
running and plenty (~100MB) free memory (page cache). Swapping kills
I/O performance because of needless disk head seeks, and thus all
recent kernels have very slow I/O (~60% of possible speed).
While at benchmarking, I have also tested 2.3.42 which is the last
kernel before elevator rewrite (Hi Andrea! :))
-------Sequential Output-------- ---Sequential Input-- --Random--
-Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
2.3.42 400 20978 97.2 22519 22.2 9302 12.7 18860 61.5 21020 20.4 114.8 1.3
^^^^
Numbers for read/write are almost same as in my experiment (which is
to say that VM subsytem worked OK in 2.3.42, at least for common
memory configurations :)), but there is a measurable difference in a
rewrite case. Old elevator allowed ~9.5MB/s rewriting speed, while
with new code it drops to ~7.5MB/s.
Question for Andrea: is it possible to get back to the old speeds with
tha new elevator code, or is the speed drop unfortunate effect of the
"non-starvation" logic, and thus can't be cured?
Doing that same rewrite test under old and new kernels reveals that in
2.3.42 disk is completely quiet while rewriting, while in the 99-pre
series it makes very loud and scary noise. Could that still be a bug
somewhere in the elevator?
Regards,
-- Zlatko- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Tue May 23 2000 - 21:00:21 EST