Re: [PATCH] improve the performance of large sequential write NFS workloads

[Steve Rago <sar@nec-labs.com>]

On Tue, 2009-12-22 at 13:25 +0100, Jan Kara wrote:
> > I originally spent several months playing with the balance_dirty_pages
> > algorithm.  The main drawback is that it affects more than the inodes
> > that the caller is writing and that the control of what to do is too
>   Can you be more specific here please?

Sure;  balance_dirty_pages() will schedule writeback by the flusher
thread once the number of dirty pages exceeds dirty_background_ratio.
The flusher thread calls writeback_inodes_wb() to flush all dirty inodes
associated with the bdi.  Similarly, the process dirtying the pages will
call writeback_inodes_wbc() when it's bdi threshold has been exceeded.
The first problem is that these functions process all dirty inodes with
the same backing device, which can lead to excess (duplicate) flushing
of the same inode.  Second, there is no distinction between pages that
need to be committed and pages that have commits pending in
NR_UNSTABLE_NFS/BDI_RECLAIMABLE (a page that has a commit pending won't
be cleaned any faster by sending more commits).  This tends to overstate
the amount of memory that can be cleaned, leading to additional commit
requests.  Third, these functions generate a commit for each set of
writes they do, which might not be appropriate.  For background writing,
you'd like to delay the commit as long as possible.

[snip]

> > 
> > Part of the patch does implement a heuristic write-behind.  See where
> > nfs_wb_eager() is called.
>   I believe that if we had per-bdi dirty_background_ratio and set it low
> for NFS's bdi, then the write-behind logic would not be needed
> (essentially the flusher thread should submit the writes to the server
> early).
> 
> 								Honza

Maybe so, but you still need something to prevent the process that is
dirtying pages from continuing, because a process can always write to
memory faster than writing to disk/network, so the flusher won't be able
to keep up.

Steve