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    SubjectRe: [PATCH] kswapd: avoid unnecessary rebalance after an unsuccessful balancing
    On Fri, Jul 29, 2011 at 11:23:10PM +0800, Alex Shi wrote:
    > In commit 215ddd66, Mel Gorman said kswapd is better to sleep after a
    > unsuccessful balancing if there is tighter reclaim request pending in
    > the balancing. In this scenario, the 'order' and 'classzone_idx'
    > that are checked for tighter request judgment is incorrect, since they
    > aren't the one kswapd should read from new pgdat, but the last time pgdat
    > value for just now balancing. Then kswapd will skip try_to_sleep func
    > and rebalance the last pgdat request. It's not our expected behavior.
    > So, I added new variables to distinguish the returned order/classzone_idx
    > from last balancing, that can resolved above issue in that scenario.

    I'm afraid this changelog is very difficult to read and I do not see
    what problem you are trying to solve and I do not see what this patch
    might solve.

    When balance_pgdat() returns with a lower classzone or order, the values
    stored in pgdat are not re-read and instead it tries to go to sleep
    based on the starting request. Something like;

    1. Read pgdat request A (classzone_idx, order)
    2. balance_pgdat()
    3. During pgdat, a new pgdat request B (classzone_idx, order) is placed
    4. balance_pgdat() returns but failed so classzone_idx is lower
    5. Try to sleep based on pgdat request A

    i.e. pgdat request B is not read and there is a comment explaining
    why pgdat request B is not read after balance_pgdat() fails.

    This patch adds some variables that might improve the readability
    for some people but otherwise I can't see what problem is being
    fixed. What did I miss?

    Mel Gorman
    SUSE Labs

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