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

[Andreas Hirstius]

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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