Re: [PATCH BUGFIX 1/3] null_blk: set a separate timer for each command

From: Jens Axboe
Date: Mon Nov 02 2015 - 11:15:01 EST

On 11/02/2015 07:31 AM, Paolo Valente wrote:
For the Timer IRQ mode (i.e., when command completions are delayed),
there is one timer for each CPU. Each of these timers
. has a completion queue associated with it, containing all the
command completions to be executed when the timer fires;
. is set, and a new completion-to-execute is inserted into its
completion queue, every time the dispatch code for a new command
happens to be executed on the CPU related to the timer.

This implies that, if the dispatch of a new command happens to be
executed on a CPU whose timer has already been set, but has not yet
fired, then the timer is set again, to the completion time of the
newly arrived command. When the timer eventually fires, all its queued
completions are executed.

This way of handling delayed command completions entails the following
problem: if more than one command completion is inserted into the
queue of a timer before the timer fires, then the expiration time for
the timer is moved forward every time each of these completions is
enqueued. As a consequence, only the last completion enqueued enjoys a
correct execution time, while all previous completions are unjustly
delayed until the last completion is executed (and at that time they
are executed all together).

Specifically, if all the above completions are enqueued almost at the
same time, then the problem is negligible. On the opposite end, if
every completion is enqueued a while after the previous completion was
enqueued (in the extreme case, it is enqueued only right before the
timer would have expired), then every enqueued completion, except for
the last one, experiences an inflated delay, proportional to the number
of completions enqueued after it. In the end, commands, and thus I/O
requests, may be completed at an arbitrarily lower rate than the
desired one.

This commit addresses this issue by replacing per-CPU timers with
per-command timers, i.e., by associating an individual timer with each

Functionally the patch looks fine. My only worry is that a timer per command would be an unnecessary slowdown compared to pushing one timer forward. The problem should be fixable by still doing that, just maintaining next-expire instead. Maybe something that would still roughly be precise enough, while still getting some completion batching going? Or maybe that would be slower, and the individual timers are still better.


Jens Axboe

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at