On 2012-08-08 11:06 Shaohua Li <s...@kernel.org> Wrote:
>2012/8/8 Jianpeng Ma <majianp...@gmail.com>:
>> I think there are three problems about handling plug in blk_queue_bio():
>> 1:if request_count >= BLK_MAX_REQUEST_COUNT, avoid unnecessary
>> plug->should_sort judge.
>this makes sense, though not a big deal, nice to fix it.
Thanks
>
>> 2:Only two device can trace plug.
>I didn't get the point, can you have more details?
>>if (plug) {
>> /*
>> * If this is the first request added after a plug, fire
>> * of a plug trace. If others have been added before, check
>> * if we have multiple devices in this plug. If so, make a
>> * note to sort the list before dispatch.
>> */
>> if (list_empty(&plug->list))
>> trace_block_plug(q);
>> else {
>> if (!plug->should_sort) {
>> struct request *__rq;
>> __rq = list_entry_rq(plug->list.prev);
>> if (__rq->q != q)
>> plug->should_sort = 1;
>> }
>> if (request_count >= BLK_MAX_REQUEST_COUNT) {
>> blk_flush_plug_list(plug, false);
>> trace_block_plug(q);
The code only trace two point;
A: list_empty(&plug->list)
B: request_count >= BLK_MAX_REQUEST_COUNT). it's the same like A which
plug->list is empty.
Suppose:
1;reqA-deviceA firstly come, it will call trace_block_plug because the
list_empty(plug->list) is true.
2:reqB-deviceB comed, attempt_plug_merge will failed because not
deviceB-request-queue.But it'll not to call trace_block_plug.
But call blk_flush_plug_list,it will trace_block_unplug all request_queue.
>
>> 3:When exec blk_flush_plug_list,it use list_sort which has
>> O(nlog(n)) complexity. When insert and sort, it only O(n) complexity.
>but now you do the list iterator for every request, so it's O(n*n)?
>The plug list is unlikely too long, so I didn't worry about the time
>spending on list sort.
Sorry, it's my fault.
