Re: [RFC] page-writeback: move indoes from one superblock together

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Sep 24 2009, Wu Fengguang wrote:
> On Thu, Sep 24, 2009 at 02:54:20PM +0800, Li, Shaohua wrote:
> > __mark_inode_dirty adds inode to wb dirty list in random order. If a disk has
> > several partitions, writeback might keep spindle moving between partitions.
> > To reduce the move, better write big chunk of one partition and then move to
> > another. Inodes from one fs usually are in one partion, so idealy move indoes
> > from one fs together should reduce spindle move. This patch tries to address
> > this. Before per-bdi writeback is added, the behavior is write indoes
> > from one fs first and then another, so the patch restores previous behavior.
> > The loop in the patch is a bit ugly, should we add a dirty list for each
> > superblock in bdi_writeback?
> > 
> > Test in a two partition disk with attached fio script shows about 3% ~ 6%
> > improvement.
> 
> A side note: given the noticeable performance gain, I wonder if it
> deserves to generalize the idea to do whole disk location ordered
> writeback. That should benefit many small file workloads more than
> 10%. Because this patch only sorted 2 partitions and inodes in 5s
> time window, while the below patch will roughly divide the disk into
> 5 areas and sort inodes in a larger 25s time window.
> 
>         http://lkml.org/lkml/2007/8/27/45
> 
> Judging from this old patch, the complexity cost would be about 250
> lines of code (need a rbtree).

First of all, nice patch, I'll add it to the current tree. I too was
pondering using an rbtree for sb+dirty_time insertion and extraction.
But for 100 inodes or less, I bet that just doing the re-sort in
writeback time ends up being cheaper on the CPU cycle side.

-- 
Jens Axboe