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    SubjectRe: RFC - how to balance Dirty+Writeback in the face of slow writeback.
    On Thu, 17 Aug 2006 09:21:51 -0400
    Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

    > On Thu, 2006-08-17 at 13:59 +1000, Neil Brown wrote:
    > > On Tuesday August 15, akpm@osdl.org wrote:
    > > > > When Dirty hits 0 (and Writeback is theoretically 80% of RAM)
    > > > > balance_dirty_pages will no longer be able to flush the full
    > > > > 'write_chunk' (1.5 times number of recent dirtied pages) and so will
    > > > > spin in a loop calling blk_congestion_wait(WRITE, HZ/10), so it isn't
    > > > > a busy loop, but it won't progress.
    > > >
    > > > This assumes that the queues are unbounded. They're not - they're limited
    > > > to 128 requests, which is 60MB or so.
    > >
    > > Ahhh... so the limit on the requests-per-queue is an important part of
    > > write-throttling behaviour. I didn't know that, thanks.
    > >
    > > fs/nfs doesn't seem to impose a limit. It will just allocate as many
    > > as you ask for until you start running out of memory. I've seen 60%
    > > of memory (10 out of 16Gig) in writeback for NFS.
    > >
    > > Maybe I should look there to address my current issue, though imposing
    > > a system-wide writeback limit seems safer.
    >
    > Exactly how would a request limit help? All that boils down to is having
    > the VM monitor global_page_state(NR_FILE_DIRTY) versus monitoring
    > global_page_state(NR_FILE_DIRTY)+global_page_state(NR_WRITEBACK).
    >

    I assume that if NFS is not limiting its NR_WRITEBACK consumption and block
    devices are doing so, we could get in a situation where NFS hogs all of the
    fixed-size NR_DIRTY+NR_WRITEBACK resource at the expense of concurrent
    block-device-based writeback.

    Perhaps. The top-level poll-the-superblocks writeback loop might tend to
    prevent that from happening. But if applications were doing a lot of
    superblock-specific writeback (fdatasync,
    sync_file_range(SYNC_FILE_RANGE_WRITE), etc) then unfairness might occur.
    -
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