Re: [PATCH v2] mm: implement write-behind policy for sequential file writes

[Konstantin Khlebnikov]

On 24/09/2019 10.39, Dave Chinner wrote:
> On Mon, Sep 23, 2019 at 06:06:46PM +0300, Konstantin Khlebnikov wrote:
> > On 23/09/2019 17.52, Tejun Heo wrote:
> > > Hello, Konstantin.
> > >
> > > On Fri, Sep 20, 2019 at 10:39:33AM +0300, Konstantin Khlebnikov wrote:
> > > > With vm.dirty_write_behind 1 or 2 files are written even faster and
> > >
> > > Is the faster speed reproducible?  I don't quite understand why this
> > > would be.
> >
> > Writing to disk simply starts earlier.
>
> Stupid question: how is this any different to simply winding down
> our dirty writeback and throttling thresholds like so:
>
> # echo $((100 * 1000 * 1000)) > /proc/sys/vm/dirty_background_bytes
>
> to start background writeback when there's 100MB of dirty pages in
> memory, and then:
>
> # echo $((200 * 1000 * 1000)) > /proc/sys/vm/dirty_bytes
>
> So that writers are directly throttled at 200MB of dirty pages in
> memory?
>
> This effectively gives us global writebehind behaviour with a
> 100-200MB cache write burst for initial writes.

Global limits affect all dirty pages including memory-mapped and
randomly touched. Write-behind aims only into sequential streams.

> ANd, really such strict writebehind behaviour is going to cause all
> sorts of unintended problesm with filesystems because there will be
> adverse interactions with delayed allocation. We need a substantial
> amount of dirty data to be cached for writeback for fragmentation
> minimisation algorithms to be able to do their job....

I think most sequentially written files never change after close.
Except of knowing final size of huge files (>16Mb in my patch)
there should be no difference for delayed allocation.

Probably write behind could provide hint about streaming pattern:
pass something like "MSG_MORE" into writeback call.

> Cheers,
>
> Dave.