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SubjectSerious performance degradation on a RAID with kernel 2.6.10-bk7 and later
Hi,


We have a rx4640 with 3x 3Ware 9500 SATA controllers and 24x WD740GD HDD
in a software RAID0 configuration (using md).
With kernel 2.6.11 the read performance on the md is reduced by a factor
of 20 (!!) compared to previous kernels.
The write rate to the md doesn't change!! (it actually improves a bit).

The config for the kernels are basically identical.

Here is some vmstat output:

kernel 2.6.9: ~1GB/s read
procs memory swap io
system cpu
r b swpd free buff cache si so bi bo in cs us sy wa id
1 1 0 12672 6592 15914112 0 0 1081344 56 15719 1583 0 11 14 74
1 0 0 12672 6592 15915200 0 0 1130496 0 15996 1626 0 11 14 74
0 1 0 12672 6592 15914112 0 0 1081344 0 15891 1570 0 11 14 74
0 1 0 12480 6592 15914112 0 0 1081344 0 15855 1537 0 11 14 74
1 0 0 12416 6592 15914112 0 0 1130496 0 16006 1586 0 12 14 74


kernel 2.6.11: ~55MB/s read
procs memory swap io
system cpu
r b swpd free buff cache si so bi bo in cs us sy wa id
1 1 0 24448 37568 15905984 0 0 56934 0 5166 1862 0 1 24 75
0 1 0 20672 37568 15909248 0 0 57280 0 5168 1871 0 1 24 75
0 1 0 22848 37568 15907072 0 0 57306 0 5173 1874 0 1 24 75
0 1 0 25664 37568 15903808 0 0 57190 0 5171 1870 0 1 24 75
0 1 0 21952 37568 15908160 0 0 57267 0 5168 1871 0 1 24 75


Because the filesystem might have an impact on the measurement, "dd" on /dev/md0
was used to get information about the performance.
This also opens the possibility to test with block sizes larger than the page size.
And it appears that the performance with kernel 2.6.11 is closely
related to the block size.
For example if the block size is exactly a multiple (>2) of the page
size the performance is back to ~1.1GB/s.
The general behaviour is a bit more complicated:

1. bs <= 1.5 * ps : ~27-57MB/s (differs with ps)
2. bs > 1.5 * ps && bs < 2 * ps : rate increases to max. rate
3. bs = n * ps ; (n >= 2) : ~1.1GB/s (== max. rate)
4. bs > n * ps && bs < ~(n+0.5) * ps ; (n > 2) : ~27-70MB/s (differs
with ps)
5. bs > ~(n+0.5) * ps && bs < (n+1) * ps ; (n > 2) : increasing rate
in several, more or
less, distinct steps (e.g. 1/3 of max. rate and then 2/3 of max
rate for 64k pages)
I've tested all four possible page sizes on Itanium (4k, 8k, 16k and 64k) and the pattern is
always the same!!

With kernel 2.6.9 (any kernel before 2.6.10-bk6) the read rate is always at ~1.1GB/s,
independent of the block size.


This simple patch solves the problem, but I have no idea of possible side-effects ...

--- linux-2.6.12-rc2_orig/mm/filemap.c 2005-04-04 18:40:05.000000000 +0200
+++ linux-2.6.12-rc2/mm/filemap.c 2005-04-20 10:27:42.000000000 +0200
@@ -719,7 +719,7 @@
index = *ppos >> PAGE_CACHE_SHIFT;
next_index = index;
prev_index = ra.prev_page;
- last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
+ last_index = (*ppos + desc->count + PAGE_CACHE_SIZE) >> PAGE_CACHE_SHIFT;
offset = *ppos & ~PAGE_CACHE_MASK;

isize = i_size_read(inode);
--- linux-2.6.12-rc2_orig/mm/readahead.c 2005-04-04 18:40:05.000000000 +0200
+++ linux-2.6.12-rc2/mm/readahead.c 2005-04-20 18:37:04.000000000 +0200
@@ -70,7 +70,7 @@
*/
static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
{
- unsigned long newsize = roundup_pow_of_two(size);
+ unsigned long newsize = size;

if (newsize <= max / 64)
newsize = newsize * newsize;


In order to keep this mail short, I've created a webpage that contains
all the detailed information and some plots:
http://www.cern.ch/openlab-debugging/raid


Regards,

Andreas Hirstius


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