RE: Direct io on block device has performance regression on 2.6.x kernel

[Chen, Kenneth W]

Andrew Morton wrote Wednesday, March 09, 2005 6:26 PM
> What does "1/3 of the total benchmark performance regression" mean?  One
> third of 0.1% isn't very impressive.  You haven't told us anything at all
> about the magnitude of this regression.

2.6.9 kernel is 6% slower compare to distributor's 2.4 kernel (RHEL3).  Roughly
2% came from storage driver (I'm not allowed to say anything beyond that, there
is a fix though).

2% came from DIO.

The rest of 2% is still unaccounted for.  We don't know where.

> How much system time?  User time?  All that stuff.
20.5% in the kernel, 79.5% in user space.


> But the first thing to do is to work out where the cycles are going to.
You've seen the profile.  That's where all the cycle went.


> Also, I'm rather peeved that we're hearing about this regression now rather
> than two years ago.  And mystified as to why yours is the only group which
> has reported it.

2.6.X kernel has never been faster than the 2.4 kernel (RHEL3).  At one point
of time, around 2.6.2, the gap is pretty close, at around 1%, but still slower.
Around 2.6.5, we found global plug list is causing huge lock contention on
32-way numa box.  That got fixed in 2.6.7.  Then comes 2.6.8 which took a big
dip at close to 20% regression.  Then we fixed 17% regression in the scheduler
(fixed with cache_decay_tick).  2.6.9 is the last one we measured and it is 6%
slower.  It's a constant moving target, a wild goose to chase.

I don't know why other people have not reported the problem, perhaps they
haven't got a chance to run transaction processing db workload on 2.6 kernel.
Perhaps they have not compared, perhaps they are working on the same problem.
I just don't know.

- Ken


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