Hi,
First of all, I am a big fan of the CFQ I/O scheduler. Thank you for your contribution. :-)
I have a question about the below CFQ queue preemption condition in cfq_should_preempt().
/*
* So both queues are sync. Let the new request get disk time if
* it's a metadata request and the current queue is doing regular IO.
*/
if ((rq->cmd_flags & REQ_PRIO) && !cfqq->prio_pending)
return true;
Normally, REQ_PRIO flag is used for READ requests for reading metadata, so then, the CFQ
queue containing "rq" must be a SYNC queue and cfqq is also a SYNC queue because we
already passed through the below condition. Therefore, the comment "So both queues are sync"
is TRUE.
if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
return true;
But, I am wordering about the case of that an asynchronous WRITE request with REQ_PRIO
is inserted into the request queue. Actually, ext4 metadata are submitted as asynchronous
WRITE requests with REQ_PRIO by kworker. In this case, asynchronous WRITE requests can
preempt synchronous READ requests. Anyways, I think ext4 metadata write should have
priority for better filesystem call responsiveness, but the comment in the source code,
"So both queues are sync", makes me a little confused.
Could you explain about the meaning of the comment in the source code?
Best Regards,