Re: Recent kernel "mount" slow

Jeff Chua
2012-11-28T13:05:30Z

On Wed, Nov 28, 2012 at 4:33 PM, Jens Axboe <axboe@kernel.dk> wrote:
> On 2012-11-28 04:57, Mikulas Patocka wrote:
>>
>>
>> On Tue, 27 Nov 2012, Jens Axboe wrote:
>>
>>> On 2012-11-27 11:06, Jeff Chua wrote:
>>>> 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.
>>>
>>> Excellent
>>>
>>>>> 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.
>>>
>>> Makes sense, it's 2 synchronize_sched() instead of 3. So it doesn't fix
>>> the real issue, which is having to do synchronize_sched() in the first
>>> place.
>>>
>>>> # 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.
>>>
>>> I wonder how many of them are due to changing to the same block size.
>>> Does the below patch make a difference?
>>
>> This patch is wrong because you must check if the device is mapped while
>> holding bdev->bd_block_size_semaphore (because
>> bdev->bd_block_size_semaphore prevents new mappings from being created)
>
> No it doesn't. If you read the patch, that was moved to i_mmap_mutex.
>
>> I'm sending another patch that has the same effect.
>>
>>
>> Note that ext[234] filesystems set blocksize to 1024 temporarily during
>> mount, so it doesn't help much (it only helps for other filesystems, such
>> as jfs). For ext[234], you have a device with default block size 4096, the
>> filesystem sets block size to 1024 during mount, reads the super block and
>> sets it back to 4096.
>
> That is true, hence I was hesitant to think it'll actually help. In any
> case, basically any block device will have at least one blocksize
> transitioned when being mounted for the first time. I wonder if we just
> shouldn't default to having a 4kb soft block size to avoid that one,
> though it is working around the issue to some degree.

I tested on reiserfs. It helped. 0.012s as in 3.6.0, but as Mikulas
mentioned, it didn't really improve much for ext2.

Jeff.