I have question about performance on a generic block device
that I have written. It's very simple device. All the block device does
is register a make_request function in addtion to the normal
blkdev operations.
When a buffer header is received in make_request function it maps the
buffer header to a new device by changing it's b_rdev to the
new device number. The make request function then returns
1 allowing the kernel to send the buffer header to the correct
device.
Everthing seems to work fine as long as I working with one minor
number. For instance, redirecting buffer_headers that are for
my device say with major 212 and minor 0 to scsi device /dev/sda1.
It works great I get the same read and write performance as if I
was writting directly to /dev/sda1.
When I try doing 2 IO streams lets say from major 212 minor 0 to
scsi device /dev/sda1 and major 212 and minor 1 to scsi device
/dev/sdb1. The read and write performance is half of what
it was when I was reading and writting to one device. In
other words the buffer headers seem to be getting serialized
thru my device.
When I try doing 2 IO streams directly to /dev/sda1 and /dev/sdb1
the aggergate banwidth of both IO streams is double of what it was
when doing the same test to one device. This is how I would expect
my generic block device to act.
I have included my make request function.
int
test_make_request( request_queue_t * queue, int rw , struct buffer_head *bh )
{
kdev_t rdev = 0;
#if CONFIG_HIGHMEM
bh = create_bounce(rw,bh);
#endif
/* Map to new device using map array */
/* the map array contains the read device number */
rdev = map[MINOR( bh->b_rdev ) ] ;
if ( rdev == 0 )
{
PERR(" Did not find correct device ");
buffer_IO_error(bh);
return 0;
}
/* redirect buffer header to new device */
bh->b_rdev = rdev ;
/* request buffer header be sent to new device */
return 1;
}
thats it. I'm not holding any locks and I'm not sleeping I do not
understand why the performance for this block device is so poor.
Is there possibly something else that needs to be done at time
of calling register_blkdev? Do I need to register a gendisk
structure?
Thank You
don jessup
donjay72@yahoo.com
=====
Don Jessup
Asaca/Shibasoku Corp. of America
400 Corporate Circle, Unit G
Golden, CO 80401
303-278-1111 X232
donj@asaca.com
http://www.asaca.com
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