Messages in this thread | | | Date | Mon, 17 Sep 2012 12:07:59 +0200 | From | Ingo Molnar <> | Subject | Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets - bisected |
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* Mike Galbraith <efault@gmx.de> wrote:
> On Sun, 2012-09-16 at 12:57 -0700, Linus Torvalds wrote: > > On Sat, Sep 15, 2012 at 9:35 PM, Mike Galbraith <efault@gmx.de> wrote: > > > > > > Oh, while I'm thinking about it, there's another scenario that could > > > cause the select_idle_sibling() change to affect pgbench on largeish > > > packages, but it boils down to preemption odds as well. > > > > So here's a possible suggestion.. > > > > Let's assume that the scheduler code to find the next idle CPU on the > > package is actually a good idea, and we shouldn't mess with the idea. > > We should definitely mess with the idea, as it causes some problems. > > > But at the same time, it's clearly an *expensive* idea, > > which is why you introduced the "only test a single CPU > > buddy" approach instead. But that didn't work, and you can > > come up with multiple reasons why it wouldn't work. Plus, > > quite fundamentally, it's rather understandable that "try to > > find an idle CPU on the same package" really would be a good > > idea, right? > > I would argue that it did work, it shut down the primary > source of pain, which I do not believe to be the traversal > cost, rather the bouncing. > > 4 socket 40 core + SMT Westmere box, single 30 sec tbench runs, higher is better: > > clients 1 2 4 8 16 32 64 128 > .......................................................................... > pre 30 41 118 645 3769 6214 12233 14312 > post 299 603 1211 2418 4697 6847 11606 14557
That's a very tempting speedup for a simpler and more fundamental workload than postgresql's somewhat weird user-space spinlocks that burn CPU time in user-space instead of blocking/waiting on a futex.
IIRC mysql does this properly and outperforms postgresql on this benchmark, in an apples-to-apples configuration?
> 10x at 1 pair shouldn't be traversal, the whole box is > otherwise idle. We'll do a lot more (ever more futile) > traversal as load increases, but at the same time, our futile > attempts fail more frequently, so we shoot ourselves in the > foot less frequently. > > The down side is (appears to be) that I also shut down some > ~odd case preemption salvation, salvation that only large > packages will receive. > > The problem as I see it is that we're making light tasks _too_ > mobile, turning an optimization into a pessimization for light > tasks. For longer running tasks this mobility within a large > package isn't such a big deal, but for fast movers, it hurts a > lot.
There's not enough time to resolve this for v3.6, so I agree with the revert - would you be willing to post a v2 of your original patch? I really think we want your tbench speedups, quite a few real-world messaging applications use the tbench patterns of scheduling.
Thanks,
Ingo
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