Re: [PATCH 0/7] Per-bdi writeback flusher threads v20

[Wu Fengguang]

On Tue, Sep 22, 2009 at 07:30:55PM +0800, Jan Kara wrote:
> On Tue 22-09-09 18:13:35, Wu Fengguang wrote:
> > Yes a more general solution would help. I'd like to propose one which
> > works in the other way round. In brief,
> > (1) the VFS give a large enough per-file writeback quota to btrfs;
> > (2) btrfs tells VFS "here is a (seek) boundary, stop voluntarily",
> >     before exhausting the quota and be force stopped.
> > 
> > There will be two limits (the second one is new):
> > 
> > - total nr to write in one wb_writeback invocation
> > - _max_ nr to write per file (before switching to sync the next inode)
> > 
> > The per-invocation limit is useful for balance_dirty_pages().
> > The per-file number can be accumulated across successive wb_writeback
> > invocations and thus can be much larger (eg. 128MB) than the legacy
> > per-invocation number. 
>   Actually, it doesn't make much sence to have a per-file limit in number
> of pages. I've been playing with an idea that we could have a per-file
> *time* quota. That would have an advantage that if a file generates random
> IO, we wouldn't block for longer time on it than when it generates linear
> IO.

Heh, FYI recently I tried per-file submission time quota:

        http://lkml.org/lkml/2009/9/10/54

Though I didn't take randomness of IO into account, which definitely
deserves some attention.

>   I imagine that in ->writepage we would substract from given time quota in
> wbc the time it takes to write the current page. It would need some context
> in wbc so that it is able to tell whether the IO is linear or random to
> properly account for some seek penalty but generally it seems to be
> doable...

Yeah, maybe page segments that are distant enough could be treated as "seeks".

>   Filesystems implementing ->writepages can then make decision whether they
> have enough time quota to seek to next extent and write it out or whether
> they should rather yield to other inodes...

Yeah, it's possible. VFS provides (one or more) quota info and
file systems decide when to yield.

Thanks,
Fengguang

> > The file system will only see the per-file numbers. The "max" means
> > if btrfs find the current page to be the last page in the extent,
> > it could indicate this fact to VFS by setting wbc->would_seek=1. The
> > VFS will then switch to write the next inode.
> > 
> > The benefit of early voluntarily yield is, it reduced the possibility
> > to be force stopped half way in an extent. When next time VFS returns
> > to sync this inode, it will again be honored the full 128MB quota,
> > which should be enough to cover a big fresh extent.
> 
> 								Honza
> -- 
> Jan Kara <jack@xxxxxxx>
> SUSE Labs, CR
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