On Tue, 11 Nov 2008 20:19:36 +0100 Jens Axboe <jens.axboe@xxxxxxxxxx> wrote: > 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. Hmm, but for 99.9% of the cases, the timeout of the block layer doesn't expire, the timeout rarely happens. The power-saving scheme can be applied to only 0.1%, but at the cost of the round_jiffies overhead per every request. If I understand correctly, round_jiffies() is designed for timers that will expire, such as periodic checking. The power-saving scheme nicely works for such usages. > 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. My exact argument is for switching from round_jiffies() to round_jiffies_up. But I wrote above, in the first place, the round_jiffies didn't make sense to me. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
