[Virtio-fs] [QUESTION] A performance problem for buffer write compared with 9p

[Gao Xiang]

On Thu, Aug 15, 2019 at 08:30:43AM +0800, wangyan wrote:

[]

> 
> I found that the judgement statement 'if (!TestSetPageDirty(page))' always
> true in function '__set_page_dirty_nobuffers', it will waste much time
> to mark inode dirty, no one page is dirty when write it the second time.
> The buffer write stack:
>     fuse_file_write_iter
>       ->fuse_cache_write_iter
>         ->generic_file_write_iter
>           ->__generic_file_write_iter
>             ->generic_perform_write
>               ->fuse_write_end
>                 ->set_page_dirty
>                   ->__set_page_dirty_nobuffers
> 
> The reason for 'if (!TestSetPageDirty(page))' always true may be the pdflush
> process will clean the page's dirty flags in clear_page_dirty_for_io(),
> and call fuse_writepages_send() to flush all pages to the disk of the host.
> So when the page is written the second time, it always not dirty.
> The pdflush stack for fuse:
>     pdflush
>       ->...
>         ->do_writepages
>           ->fuse_writepages
>             ->write_cache_pages         // will clear all page's dirty flags
>               ->clear_page_dirty_for_io // clear page's dirty flags
>             ->fuse_writepages_send      // write all pages to the host, but
> don't wait the result
> Why not wait for getting the result of writing back pages to the host
> before cleaning all page's dirty flags?
> 

As my understanding, I personally think there's nothing wrong with
the above process from your words above.

Thanks,
Gao Xiang

> As for 9p, pdflush will call clear_page_dirty_for_io() to clean the page's
> dirty flags. Then call p9_client_write() to write the page to the host,
> waiting for the result, and then flush the next page. In this case, buffer
> write of 9p will hit the dirty page many times before it is being write
> back to the host by pdflush process.
> The pdflush stack for 9p:
>     pdflush
>       ->...
>         ->do_writepages
>           ->generic_writepages
>             ->write_cache_pages
>               ->clear_page_dirty_for_io // clear page's dirty flags
>               ->__writepage
>                 ->v9fs_vfs_writepage
>                   ->v9fs_vfs_writepage_locked
>                     ->p9_client_write   // it will get the writing back
> page's result
> 
> 
> According to the test result, is the handling method of 9p for page writing
> back more reasonable than virtiofs?
> 
> Thanks,
> Yan Wang
>