Re: [PATCH] kswapd: avoid unnecessary rebalance after an unsuccessful balancing

On Fri, 2011-07-29 at 23:40 +0800, Mel Gorman wrote:

On Fri, Jul 29, 2011 at 11:23:10PM +0800, Alex Shi wrote:

quoted

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;

Thanks for your comments, I will use this comments style next time, list
request A, B etc. 

1. Read pgdat request A (classzone_idx, order)

Assume the order of A > 0, like is 3.

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

Another balance_pgdat() failure indicate is returned order == 0, am I
right?  If so, the next step of kswapd is not trying to sleep, but do
request A balance again.  And I thought this behavior doesn't match the
comments in kswapd. 

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?

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