Re: Serious performance degradation on a RAID with kernel 2.6.10-bk7and later

From: Andreas Hirstius
Date: Thu Apr 21 2005 - 03:45:59 EST


A small update.

Patching mm/filemap.c is not necessary in order to get the improved performance!
It's sufficient to remove roundup_pow_of_two from |get_init_ra_size ...

So a simple one-liner changes to picture dramatically.
But why ?!?!?


Andreas
|

jmerkey wrote:



For 3Ware, you need to chage the queue depths, and you will see dramatically improved performance. 3Ware can take requests
a lot faster than Linux pushes them out. Try changing this instead, you won't be going to sleep all the time waiting on the read/write
request queues to get "unstarved".


/linux/include/linux/blkdev.h

//#define BLKDEV_MIN_RQ 4
//#define BLKDEV_MAX_RQ 128 /* Default maximum */
#define BLKDEV_MIN_RQ 4096
#define BLKDEV_MAX_RQ 8192 /* Default maximum */


Jeff

Andreas Hirstius wrote:

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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