Re: [PATCH] f2fs: optimize fs_lock for better performance

[Gu Zheng <guz.fnst@cn.fujitsu.com>]

Hi Chao,
On 09/12/2013 10:40 AM, 俞超 wrote:

> Hi Gu
> 
>> -----Original Message-----
>> From: Gu Zheng [mailto:guz.fnst@cn.fujitsu.com]
>> Sent: Wednesday, September 11, 2013 1:38 PM
>> To: jaegeuk.kim@samsung.com
>> Cc: chao2.yu@samsung.com; shu.tan@samsung.com;
>> linux-fsdevel@vger.kernel.org; linux-kernel@vger.kernel.org;
>> linux-f2fs-devel@lists.sourceforge.net
>> Subject: Re: [f2fs-dev][PATCH] f2fs: optimize fs_lock for better performance
>>
>> Hi Jaegeuk, Chao,
>>
>> On 09/10/2013 08:52 AM, Jaegeuk Kim wrote:
>>
>>> Hi,
>>>
>>> At first, thank you for the report and please follow the email writing
>>> rules. :)
>>>
>>> Anyway, I agree to the below issue.
>>> One thing that I can think of is that we don't need to use the
>>> spin_lock, since we don't care about the exact lock number, but just
>>> need to get any not-collided number.
>>
>> IMHO, just moving sbi->next_lock_num++ before
>> mutex_lock(&sbi->fs_lock[next_lock])
>> can avoid unbalance issue mostly.
>> IMO, the case two or more threads increase sbi->next_lock_num in the same
>> time is really very very little. If you think it is not rigorous, change
>> next_lock_num to atomic one can fix it.
>> What's your opinion?
>>
>> Regards,
>> Gu
> 
> I did the test sbi->next_lock_num++ compare with the atomic one,
> And I found performance of them is almost the same under a small number thread racing.
> So as your and Kim's opinion, it's enough to use "sbi->next_lock_num++" to fix this issue.

Good, but it seems that your replay patch is out of format, and it's hard for Jaegeuk to merge.
I'll format it, see the following thread.

Thanks,
Gu

> 
> Thanks for the advice.
>>
>>>
>>> So, how about removing the spin_lock?
>>> And how about using a random number?
>>
>>> Thanks,
>>>
>>> 2013-09-06 (금), 09:48 +0000, Chao Yu:
>>>> Hi Kim:
>>>>
>>>>      I think there is a performance problem: when all sbi->fs_lock is
>>>> holded,
>>>>
>>>> then all other threads may get the same next_lock value from
>>>> sbi->next_lock_num in function mutex_lock_op,
>>>>
>>>> and wait to get the same lock at position fs_lock[next_lock], it
>>>> unbalance the fs_lock usage.
>>>>
>>>> It may lost performance when we do the multithread test.
>>>>
>>>>
>>>>
>>>> Here is the patch to fix this problem:
>>>>
>>>>
>>>>
>>>> Signed-off-by: Yu Chao <chao2.yu@samsung.com>
>>>>
>>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>>>>
>>>> old mode 100644
>>>>
>>>> new mode 100755
>>>>
>>>> index 467d42d..983bb45
>>>>
>>>> --- a/fs/f2fs/f2fs.h
>>>>
>>>> +++ b/fs/f2fs/f2fs.h
>>>>
>>>> @@ -371,6 +371,7 @@ struct f2fs_sb_info {
>>>>
>>>>         struct mutex fs_lock[NR_GLOBAL_LOCKS];  /* blocking FS
>>>> operations */
>>>>
>>>>         struct mutex node_write;                /* locking node
>> writes
>>>> */
>>>>
>>>>         struct mutex writepages;                /* mutex for
>>>> writepages() */
>>>>
>>>> +       spinlock_t spin_lock;                   /* lock for
>>>> next_lock_num */
>>>>
>>>>         unsigned char next_lock_num;            /* round-robin
>> global
>>>> locks */
>>>>
>>>>         int por_doing;                          /* recovery is doing
>>>> or not */
>>>>
>>>>         int on_build_free_nids;                 /* build_free_nids is
>>>> doing */
>>>>
>>>> @@ -533,15 +534,19 @@ static inline void mutex_unlock_all(struct
>>>> f2fs_sb_info *sbi)
>>>>
>>>>
>>>>
>>>>  static inline int mutex_lock_op(struct f2fs_sb_info *sbi)
>>>>
>>>>  {
>>>>
>>>> -       unsigned char next_lock = sbi->next_lock_num %
>>>> NR_GLOBAL_LOCKS;
>>>>
>>>> +       unsigned char next_lock;
>>>>
>>>>         int i = 0;
>>>>
>>>>
>>>>
>>>>         for (; i < NR_GLOBAL_LOCKS; i++)
>>>>
>>>>                 if (mutex_trylock(&sbi->fs_lock[i]))
>>>>
>>>>                         return i;
>>>>
>>>>
>>>>
>>>> -       mutex_lock(&sbi->fs_lock[next_lock]);
>>>>
>>>> +       spin_lock(&sbi->spin_lock);
>>>>
>>>> +       next_lock = sbi->next_lock_num % NR_GLOBAL_LOCKS;
>>>>
>>>>         sbi->next_lock_num++;
>>>>
>>>> +       spin_unlock(&sbi->spin_lock);
>>>>
>>>> +
>>>>
>>>> +       mutex_lock(&sbi->fs_lock[next_lock]);
>>>>
>>>>         return next_lock;
>>>>
>>>>  }
>>>>
>>>>
>>>>
>>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>>>
>>>> old mode 100644
>>>>
>>>> new mode 100755
>>>>
>>>> index 75c7dc3..4f27596
>>>>
>>>> --- a/fs/f2fs/super.c
>>>>
>>>> +++ b/fs/f2fs/super.c
>>>>
>>>> @@ -657,6 +657,7 @@ static int f2fs_fill_super(struct super_block
>>>> *sb, void *data, int silent)
>>>>
>>>>         mutex_init(&sbi->cp_mutex);
>>>>
>>>>         for (i = 0; i < NR_GLOBAL_LOCKS; i++)
>>>>
>>>>                 mutex_init(&sbi->fs_lock[i]);
>>>>
>>>> +       spin_lock_init(&sbi->spin_lock);
>>>>
>>>>         mutex_init(&sbi->node_write);
>>>>
>>>>         sbi->por_doing = 0;
>>>>
>>>>         spin_lock_init(&sbi->stat_lock);
>>>>
>>>> (END)
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>
>>
>> =
> 
> 



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