Messages in this thread | | | Date | Tue, 11 Nov 2008 20:19:36 +0100 | From | Jens Axboe <> | Subject | Re: Problems with the block-layer timeouts |
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On Tue, Nov 11 2008, Alan Stern wrote: > On Tue, 11 Nov 2008, FUJITA Tomonori wrote: > > > I don't worry about anything. I just think that these round_jiffies_up > > are pointless because they were added for the block-layer users that > > care about exact timeouts, however the block-layer doesn't export > > blk_add_timer() so the block-layer users can't control the exact time > > when the timer starts. So doing round_jiffies_up calculation per every > > request doesn't make sense for me. > > In fact the round_jiffies_up() routines were added for other users as > well as the block layer. However none of the others could be changed > until the routines were merged. Now that the routines are in the > mainline, you should see them start to be called in multiple places. > > Also, the users of the block layer _don't_ care about exact timeouts. > That's an important aspect of round_jiffies() and round_jiffies_up() -- > you don't use them if you want an exact timeout. > > The reason for using round_jiffies() is to insure that the timeout > will occur at a 1-second boundary. If several timeouts are set for > about the same time and they all use round_jiffies() or > round_jiffies_up(), then they will all occur at the same tick instead > of spread out among several different ticks during the course of that > 1-second interval. As a result, the system will need to wake up only > once to service all those timeouts, instead of waking up several > different times. It is a power-saving scheme.
I can't add anything else, can't say it any better either. The main point of using round_jiffies_up() is to align with other timers. I don't understand why you (Tomo) think that timeouts are exact? They really are not, and within the same second is quite adequate here.
-- Jens Axboe
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