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List:       netbsd-tech-kern
Subject:    Re: high load, no bottleneck
From:       Mouse <mouse () Rodents-Montreal ! ORG>
Date:       2013-09-30 18:42:50
Message-ID: 201309301842.OAA11419 () Chip ! Rodents-Montreal ! ORG
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>> Basically, if we have N pending VOP_FSYNC for a given filesystem,
>> all theses requests will be honoured on first flush, but they are
>> serialized and will be acknowledged one by one, with the cost of a
>> useless flush each time.  Am I right?
> Do you mean "all these requests *could* be honored on first flush"?
> If so, then yes, I agree.

Last flush, surely?  There may be pending operations between the first
FSYNC and the last which need to be performed before the last FSYNC is
done.

> Looking at the code in XFS that gathers barrier events, it seems to
> be drawing a DAG...

That's exactly what I'd expect: write-&-flush dependency _is_ a DAG,
with barriers being chokepoints (I forget the graph-theory term for
them) in the DAG.

					Mouse
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