Messages in this thread |  | | | Date | Fri, 7 May 1999 13:10:07 -0400 (EDT) | | From | Phillip Ezolt <> | | Subject | Re: Overscheduling DOES happen with high web server load. |
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Greg, For the record, I have been doing my performance tests on "strobe" runs of SPECWeb96. This is a somewhat representative subset of the entire suite. It runs much shorter than a full SPECWeb96 run. It allows one to quickly figure what is happening in the system, and is ideal for performance tuning.
I haven't yet tried a full run with Greg's changes, but on the strobe runs, things do get better. Response time descreases and the maximum number of Ops increases. (Both very good signs.)
I'll try a full run shortly.
--Phil
Digital/Compaq: HPSD/Benchmark Performance Engineering Phillip.Ezolt@compaq.com ezolt@perf.zko.dec.com
On Fri, 7 May 1999, Greg Lindahl wrote:
> > Ok, you are right. The real problem is we are calculating goodness > > O(A*B). > > > > A= Number of processes on the runqueue > > B= Number of times schedule is called > > This is the amount of work we are doing, but I think you're on to the > wrong solution. Phil did a test: he patched schedule() to pick the > first schedulable process instead of the best one. That dropped the > amount of time consumed in schedule() from 20% to 1%. That's the same > as reducing the work from O(A*B) to O(B). > > Why is schedule called so frequently? The thundering herd. You get a > new connection, everyone wakes up, only one gets work, and everyone > else goes back to sleep, each causing O(A) work to reschedule. M hits > per second, N processes, B=M*N. A=N. So the total work is O(M*N^2). > > These are separate problems. The thundering herd is fixed by > wake_one. The cost of scheduling is still a problem; if we had > many-cpu SMP linux boxes with high loads, all the cpus would sit > around waiting for the scheduler lock. Of course, there would be other > problems too. Since we can't fix all possible thundering herd > situations, I think we should fix the scheduler too. > > Phil, did the SpecWeb score rise with the patch? It should have. Of > course, that hacked scheduler is pretty broken... > > -- g > > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.rutgers.edu > Please read the FAQ at http://www.tux.org/lkml/ > >
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