Jens Axboe wrote:
On Sat, Oct 28 2006, Ravi Krishnamurthy wrote:
Hi all,
I have written a block driver that registers a virtual device and
routes requests to appropriate real devices after some re-mapping of
the requests. I am testing the driver by creating a filesystem on the
virtual device and copying a large number of files on to it. The test
causes the device to become unresponsive after some time. After some
debugging, I noticed that this happens only if the I/O scheduler being
used is CFQ. I have not had any trouble if the scheduler is noop,
anticipatory or deadline. The problem occurs on all the kernels I have
tested - 2.6.18-rc2, 2.6.18-rc4, 2.6.19-rc3.
The io scheduler is not obligated to recall your request handling
function, _unless_ you have no pending io at the point where
elv_next_request() returns NULL but there are things pending.
IOW, when you complete your requests you want to just recall your request handling
function. Just insert something ala:
if (elv_next_request(q))
q->request_fn(q);
when you are done completing requests.
Does that fix it?
I haven't had a chance to test this fix. A workaround I had tried was to
insert these lines at the end of the request function:
if (! elv_queue_empty(q))
blk_plug_device(q);
This worked for me. So I assume the fix you have suggested will surely
work.
I am curious to know why the problem does not occur when I am using the
anticipatory scheduler. Also, in the suggested fix, is it guaranteed that
elv_next_request() will not return NULL as long as the elevator queue is
not empty?
Thanks,
Ravi.
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