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

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

Hi Jaegeuk,

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.

Agree, but if all the locks are held, IMO, we need to balance the following
threads to wait for each not-collided number lock, though complete balance is unreachable.

> 
> So, how about removing the spin_lock?

Yeah, in this case, spin_lock is a bit heavy cost. 

> And how about using a random number?

Now NR_GLOBAL_LOCKS is 8, it seems that random can not offer an balance number as we expected.

Regards,
Gu 

> 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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