Re: Recent kernel "mount" slow

Jeff Chua
2012-11-27T10:06:17Z

On Tue, Nov 27, 2012 at 3:38 PM, Jens Axboe <axboe@kernel.dk> wrote:
> On 2012-11-27 06:57, Jeff Chua wrote:
>> On Sun, Nov 25, 2012 at 7:23 AM, Jeff Chua <jeff.chua.linux@gmail.com> wrote:
>>> On Sun, Nov 25, 2012 at 5:09 AM, Mikulas Patocka <mpatocka@redhat.com> 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.