Re: [PATCH RFC] Btrfs: improve multi-thread buffer read

[Liu Bo <liubo2009@cn.fujitsu.com>]

On 07/11/2012 02:58 AM, Josef Bacik wrote:

> On Tue, Jul 10, 2012 at 05:27:59AM -0600, Liu Bo wrote:
>> While testing with my buffer read fio jobs[1], I find that btrfs does not
>> perform well enough.
>>
>> Here is a scenario in fio jobs:
>>
>> We have 4 threads, "t1 t2 t3 t4", starting to buffer read a same file,
>> and all of them will race on add_to_page_cache_lru(), and if one thread
>> successfully puts its page into the page cache, it takes the responsibility
>> to read the page's data.
>>
>> And what's more, reading a page needs a period of time to finish, in which
>> other threads can slide in and process rest pages:
>>
>>      t1          t2          t3          t4
>>    add Page1
>>    read Page1  add Page2
>>      |         read Page2  add Page3
>>      |            |        read Page3  add Page4
>>      |            |           |        read Page4
>> -----|------------|-----------|-----------|--------
>>      v            v           v           v
>>     bio          bio         bio         bio
>>
>> Now we have four bios, each of which holds only one page since we need to
>> maintain consecutive pages in bio.  Thus, we can end up with far more bios
>> than we need.
>>
>> Here we're going to
>> a) delay the real read-page section and
>> b) try to put more pages into page cache.
>>
>> With that said, we can make each bio hold more pages and reduce the number
>> of bios we need.
>>
>> Here is some numbers taken from fio results:
>>          w/o patch                 w patch
>>        -------------  --------  ---------------
>> READ:    745MB/s        +32%       987MB/s
>>
> 
> Um, I have this in btrfs-next
> 
> Btrfs: use large extent range for read and its endio
> 
> that seems to do the same thing, did you not want to do that anymore?  Thanks,
> 



I'm still hard working on that patchset. :)

Although the patchset is well worthy of testing, it is not good enough for btrfs upstream.

While doing some tuning work on it, I realized that I could make this improvement without
the help of rwlock extent state stuff, so I made this smaller and cleaner patch for upstream
so that we could gain some performance here first.

thanks,
liubo


> Josef
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