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SubjectRe: [tbench regression fixes]: digging out smelly deadmen.
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On Sat, 2008-10-11 at 20:13 +0200, Mike Galbraith wrote:
> On Sat, 2008-10-11 at 16:39 +0200, Peter Zijlstra wrote:
>
> > That said, we can probably still avoid the division for the top level
> > stuff, because the sum of the top level weights is still invariant
> > between all tasks.
>
> Less math would be nice of course...
>
> > I'll have a stab at doing so... I initially didn't do this because my
> > first try gave some real ugly code, but we'll see - these numbers are a
> > very convincing reason to try again.
>
> ...but the numbers I get on Q6600 don't pin the tail on the math donkey.

Since I showed the rest of my numbers, I may as well show freshly
generated oltp numbers too. Chart attached. 2.6.27.rev is 2.6.27 with
weight/asym changes reverted.

Data:

read/write requests/sec per client count
1 2 4 8 16 32 64 128 256
2.6.26.6.mysql 7978 19856 37238 36652 34399 33054 31608 27983 23411
2.6.27.mysql 9618 18329 37128 36504 33590 31846 30719 27685 21299
2.6.27.rev.mysql 10944 19544 37349 36582 33793 31744 29161 25719 21026
2.6.28.git.mysql 9518 18031 30418 33571 33330 32797 31353 29139 25793

2.6.26.6.pgsql 14165 27516 53883 53679 51960 49694 44377 35361 32879
2.6.27.pgsql 14146 27519 53797 53739 52850 47633 39976 30552 28741
2.6.27.rev.pgsql 14168 27561 53973 54043 53150 47900 39906 31987 28034
2.6.28.git.pgsql 14404 28318 55124 55010 55002 54890 53745 53519 52215

Less cycles used on math is still better of course, but my box seems to
care a lot more about preemption timing than math, regardless of load.

-Mike

Aside: Don't pay too much attention to mysql client < cores (4)
numbers, these jitter considerably.

Aside2: Inquiring minds may wonder about pgsql numbers - preempting
holder of nasty userland spinlock hurts like heck. It's triggered by
short term fairness, too much, and you pay. Easy to cure, but you pay
for the cure too. Moral of story is if you're running heavily loaded
server, turn preemption knobs, you'll lose peak, but scale better.
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