Re: [PATCH v2 9/9] btrfs: Replace thread_pool_size with workqueue default value

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

于 2013年09月13日 11:15, Qu Wenruo 写道:
> 于 2013年09月13日 09:47, Liu Bo 写道:
>> On Thu, Sep 12, 2013 at 04:08:24PM +0800, Qu Wenruo wrote:
>>> The original btrfs_workers uses the fs_info->thread_pool_size as the
>>> max_active, and the previous patches followed this way.
>>>
>>> But the kernel workqueue has the default value(0) for workqueue,
>>> and workqueue itself has some threshold mechanism to prevent creating
>>> too many threads, so we should use the default value.
>>>
>>> Since the thread_pool_size algorithm is not used, related codes should
>>> also be changed.
>> Ohh, I should have seen this mail first before commenting 'max_active'.
>>
>> I think that some tuning work should be done on this part, according to
>> my tests, setting max_active=0 will create ~258 worker helpers
>> (kworker/uX:X if you set WQ_UNBOUND), this may cause too many context
>> switches which will have an impact on performance in some cases.
> Yes, but the default number when using max_active=0 should be 256
> (half of the WQ_DFL_ACTIVE).
>
> Also in my test(single thread), the performance and CPU usage does not 
> change too much.
> So it seems that in this situation, the kernel has some control on 
> creating
> kthreads.
>
> Still further max_active tunning is still quiet good.
>
> Qu
Sorry for the last reply, according to the workqueue source code,
the unbound workqueue will continually create new thread if needed.

So the default value is not good for this situation.
Also there are so many wq using the unbound wq, the old thread_pool_size 
is too small.
(Maybe change some wq to bounded would be a good idea?)

It would be quite nice if you have any good idea or advice about the
tunning about max_active.

Qu

>>
>> -liubo
>>
>>> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
>>> ---
>>>   fs/btrfs/disk-io.c | 12 +++++++-----
>>>   fs/btrfs/super.c   |  3 +--
>>>   2 files changed, 8 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>>> index a61e1fe..0446d27 100644
>>> --- a/fs/btrfs/disk-io.c
>>> +++ b/fs/btrfs/disk-io.c
>>> @@ -750,9 +750,11 @@ int btrfs_bio_wq_end_io(struct btrfs_fs_info 
>>> *info, struct bio *bio,
>>>     unsigned long btrfs_async_submit_limit(struct btrfs_fs_info *info)
>>>   {
>>> -    unsigned long limit = min_t(unsigned long,
>>> -                    info->thread_pool_size,
>>> -                    info->fs_devices->open_devices);
>>> +    unsigned long limit;
>>> +    limit = info->thread_pool_size ?
>>> +        min_t(unsigned long, info->thread_pool_size,
>>> +              info->fs_devices->open_devices) :
>>> +        info->fs_devices->open_devices;
>>>       return 256 * limit;
>>>   }
>>>   @@ -2191,8 +2193,8 @@ int open_ctree(struct super_block *sb,
>>>       INIT_RADIX_TREE(&fs_info->reada_tree, GFP_NOFS & ~__GFP_WAIT);
>>>       spin_lock_init(&fs_info->reada_lock);
>>>   -    fs_info->thread_pool_size = min_t(unsigned long,
>>> -                      num_online_cpus() + 2, 8);
>>> +    /* use the default value of kernel workqueue */
>>> +    fs_info->thread_pool_size = 0;
>>>         INIT_LIST_HEAD(&fs_info->ordered_roots);
>>>       spin_lock_init(&fs_info->ordered_root_lock);
>>> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
>>> index 63e653c..ccf412f 100644
>>> --- a/fs/btrfs/super.c
>>> +++ b/fs/btrfs/super.c
>>> @@ -898,8 +898,7 @@ static int btrfs_show_options(struct seq_file 
>>> *seq, struct dentry *dentry)
>>>       if (info->alloc_start != 0)
>>>           seq_printf(seq, ",alloc_start=%llu",
>>>                  (unsigned long long)info->alloc_start);
>>> -    if (info->thread_pool_size !=  min_t(unsigned long,
>>> -                         num_online_cpus() + 2, 8))
>>> +    if (info->thread_pool_size)
>>>           seq_printf(seq, ",thread_pool=%d", info->thread_pool_size);
>>>       if (btrfs_test_opt(root, COMPRESS)) {
>>>           if (info->compress_type == BTRFS_COMPRESS_ZLIB)
>>> -- 
>>> 1.8.4
>>>
>>> -- 
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>
>


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Qu Wenruo
Development Dept.I
Nanjing Fujitsu Nanda Software Tech. Co., Ltd.(FNST)
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