Messages in this thread | | | Date | Mon, 6 May 2013 12:32:58 -0700 | Subject | Re: [GIT PULL, RFC] Full dynticks, CONFIG_NO_HZ_FULL feature | From | Linus Torvalds <> |
| |
On Mon, May 6, 2013 at 8:35 AM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: >> >> I think Linus might have referred to my 'future plans' entry:
Indeed. I feel that HPC is entirely irrelevant to anybody, *especially* HPC benchmarks. In real life, even HPC doesn't tend to have the nice behavior their much-touted benchmarks have.
So as long as the NOHZ is for HPC-style loads, then quite frankly, I don't feel it is worth it. The _only_ thing that makes it worth it is that "future plans" part where it would actually help real loads.
>> >> Interesting that HZ=1000 caused 8% overhead there. On a regular x86 server >> PC I've measured the HZ=1000 overhead to pure user-space execution to be >> around 1% (sometimes a bit less, sometimes a bit more). >> >> But even 1% is worth it. > > I believe that the difference is tick skew
Quite possibly it is also virtualization.
The VM people are the one who complain the loudest about how certain things make their performance go down the toilet. And interrupts tend to be high on that list, and unless you have hardware support for virtual timer interrupts I can easily see a factor of four cost or more.
And the VM people then flail around wildly to always blame everybody else. *Anybody* else than the VM overhead itself.
It also depends a lot on architecture. The ia64 people had much bigger problems with the timer interrupt than x86 ever did. Again, they saw this mainly on the HPC benchmarks, because the benchmarks were carefully tuned to have huge-page support and were doing largely irrelevant things like big LINPACK runs, and the timer irq ended up blowing their carefully tuned caches and TLB's out.
Never mind that nobody sane ever *cared*. Afaik, no real HPC load has anything like that behavior, much less anything else. But they had numbers to prove how bad it was, and it was a load with very stable numbers.
Combine the two (bad HPC benchmarks and VM), and you can make an argument for just about anything. And people have.
I am personally less than impressed with some of the benchmarks I've seen, if it wasn't clear.
Linus
| |