On Tue, 27 Nov 2012, Jeff Chua wrote: > On Tue, Nov 27, 2012 at 3:38 PM, Jens Axboe <axboe@xxxxxxxxx> wrote: > > On 2012-11-27 06:57, Jeff Chua wrote: > >> On Sun, Nov 25, 2012 at 7:23 AM, Jeff Chua <jeff.chua.linux@xxxxxxxxx> wrote: > >>> On Sun, Nov 25, 2012 at 5:09 AM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > >>>> So it's better to slow down mount. > >>> > >>> I am quite proud of the linux boot time pitting against other OS. Even > >>> with 10 partitions. Linux can boot up in just a few seconds, but now > >>> you're saying that we need to do this semaphore check at boot up. By > >>> doing so, it's inducing additional 4 seconds during boot up. > >> > >> By the way, I'm using a pretty fast SSD (Samsung PM830) and fast CPU > >> (2.8GHz). I wonder if those on slower hard disk or slower CPU, what > >> kind of degradation would this cause or just the same? > > > > It'd likely be the same slow down time wise, but as a percentage it > > would appear smaller on a slower disk. > > > > Could you please test Mikulas' suggestion of changing > > synchronize_sched() in include/linux/percpu-rwsem.h to > > synchronize_sched_expedited()? > > Tested. It seems as fast as before, but may be a "tick" slower. Just > perception. I was getting pretty much 0.012s with everything reverted. > With synchronize_sched_expedited(), it seems to be 0.012s ~ 0.013s. > So, it's good. > > > > linux-next also has a re-write of the per-cpu rw sems, out of Andrews > > tree. It would be a good data point it you could test that, too. > > Tested. It's slower. 0.350s. But still faster than 0.500s without the patch. > > # time mount /dev/sda1 /mnt; sync; sync; umount /mnt > > > So, here's the comparison ... > > 0.500s 3.7.0-rc7 > 0.168s 3.7.0-rc2 > 0.012s 3.6.0 > 0.013s 3.7.0-rc7 + synchronize_sched_expedited() > 0.350s 3.7.0-rc7 + Oleg's patch. > > > Thanks, > Jeff. OK, I'm seinding two patches to reduce mount times. If it is possible to put them to 3.7.0, put them there. Mikulas --- percpu-rwsem: use synchronize_sched_expedited Use synchronize_sched_expedited() instead of synchronize_sched() to improve mount speed. This patch improves mount time from 0.500s to 0.013s. Note: if realtime people complain about the use synchronize_sched_expedited() and synchronize_rcu_expedited(), I suggest that they introduce an option CONFIG_REALTIME or /proc/sys/kernel/realtime and turn off these *_expedited functions if the option is enabled (i.e. turn synchronize_sched_expedited into synchronize_sched and synchronize_rcu_expedited into synchronize_rcu). Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> --- include/linux/percpu-rwsem.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) Index: linux-3.7-rc7/include/linux/percpu-rwsem.h =================================================================== --- linux-3.7-rc7.orig/include/linux/percpu-rwsem.h 2012-11-28 02:41:03.000000000 +0100 +++ linux-3.7-rc7/include/linux/percpu-rwsem.h 2012-11-28 02:41:15.000000000 +0100 @@ -13,7 +13,7 @@ struct percpu_rw_semaphore { }; #define light_mb() barrier() -#define heavy_mb() synchronize_sched() +#define heavy_mb() synchronize_sched_expedited() static inline void percpu_down_read(struct percpu_rw_semaphore *p) { @@ -51,7 +51,7 @@ static inline void percpu_down_write(str { mutex_lock(&p->mtx); p->locked = true; - synchronize_sched(); /* make sure that all readers exit the rcu_read_lock_sched region */ + synchronize_sched_expedited(); /* make sure that all readers exit the rcu_read_lock_sched region */ while (__percpu_count(p->counters)) msleep(1); heavy_mb(); /* C, between read of p->counter and write to data, paired with B */ -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html