Re: [PATCH 2/2] UBIFS: start using hrtimers

[Artem Bityutskiy]

Peter Zijlstra wrote:
> On Fri, 2009-05-29 at 18:09 +0300, Artem Bityutskiy wrote:
> > From: Artem Bityutskiy <Artem.Bityutskiy@xxxxxxxxx>
> > Subject: [PATCH 2/2] UBIFS: start using hrtimers
> >
> > UBIFS uses timers for write-buffer write-back. It is not
> > crucial for us to write-back exactly on time. We are fine
> > to write-back a little earlier or later. And this means
> > we may optimize UBIFS timer so that it could be groped
> > with a close timer event, so that the CPU would not be
> > waken up just to do the write back. This is optimization
> > to lessen power consumption, which is important in
> > embedded devices UBIFS is used for.
> >
> > hrtimers have a nice feature: they are effectively range
> > timers, and we may defind the soft and hard limits for
> > it. Standard timers do not have these feature. They may
> > only be made deferrable, but this means there is effectively
> > no hard limit. So, we will better use hrtimers.
> >
> > Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@xxxxxxxxx>
> > ---
>
> Seems sane enough from a hrtimer POV, but isn't this already
> functionality that the VFS/pdflush provide?

Yeah, VFS/pdflush takes care of the page-cache and inode cache,
and dirty superblocks. But additionally to this UBIFS has its
own small buffer of (usually) 2KiB size, we call it write-buffer.
This is a very important optimization for NAND flash. And we
have a separate timer to synchronize this small write-buffer.

I was also thinking to hook to VFS, which would mean creating
a fake inode representing our write-bufffer. But this would be
very hacky.

--
Best Regards,
Artem Bityutskiy (ÐÑÑÑÐ ÐÐÑÑÑÐÐÐ)
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