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    SubjectRe: [PATCH 2/2] UBIFS: start using hrtimers
    Peter Zijlstra wrote:
    > On Fri, 2009-05-29 at 18:09 +0300, Artem Bityutskiy wrote:
    >> From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
    >> 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@nokia.com>
    >> ---
    >
    > 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 (Артём Битюцкий)
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