Re: [PATCH] deadline-iosched: don't allow aliased requests to starve others

[Jeff Moyer <jmoyer@redhat.com>]

Jeff Moyer <jmoyer@redhat.com> writes:

> Jeff Moyer <jmoyer@redhat.com> writes:
>
>> Hi,
>>
>> In running a test case that tries to trip up the kernel's AIO
>> implementation, we ran into a situation where no other I/O to the device
>> under test would be completed.  The test program spawned (in this case)
>> 100 threads, each of which performed the following in a loop:
>>
>> open file O_DIRECT
>> queue 1MB of read I/O from file using 16 iocbs
>> close file
>> repeat
>>
>> The program does NOT wait for the I/O to complete.  The file length is
>> only 4MB, meaning that you have 25 threads performing I/O on each of the
>> 4 1MB regions.
>>
>> Both deadline and cfq check for aliased requests in the sorted list of
>> I/Os, and when an alias is found, the request in the rb tree is moved to
>> the dispatch list.  So, what happens is that, with this workload, only
>> requests from this program are moved to the dispatch list, starving out
>> all other I/O.
>>
>> The attached patch fixes this problem by issuing all expired requests in
>> the aliased request handling code.  The reason I opted to issue all
>> expired requsts is because if we only service a single one, I still see
>> really awful interactivity;  an ls would take over 5 minutes to
>> complete.  With the attached patch, the ls took about 7 seconds to
>> complete.
>
> It occured to me that this doesn't solve the problem of starving WRITE
> I/O.  So, this patch fixes that as well, tested with a dd if=/dev/zero
> of=outfile bs=1M count=1 oflag=sync while.

Gah.  Forgot:

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>