Messages in this thread | | | Date | Sat, 12 Sep 2009 07:49:17 +0200 | From | Ingo Molnar <> | Subject | Re: [GIT PULL] sched/core for v2.6.32 |
| |
* Tony Luck <tony.luck@intel.com> wrote:
> On Fri, Sep 11, 2009 at 4:34 PM, Jesper Juhl <jj@chaosbits.net> wrote: > > That may be so; but most people I've ever talked to about multiple > > processes, fork, vfork and the like, have mostly assumed child-runs-first. > > That is just my personal experience. > > So I get worried when that assumption is made false. > > With multi-core cpus becoming (being?) the norm, almost all > systems are SMP now. So child and parent can surely end up > running in parallel very often. So applications that make > assumptions about child running first are going to be frequently > surprised. Aren't they?
We had parent-runs-first briefly, in v2.6.23 - this got changed by v2.6.24 - but yes, it did trigger at least one app bug that i remember (dont remember which one it was though).
We are almost two years later now - maybe it works fine now.
In any case, as a precaution i made the sched_child_runs_first sysctl knob unconditional (previously it was under CONFIG_SCHED_DEBUG).
So if an old distro is upgraded with a new kernel (and user-space is not updated), it can be worked around by putting this into /etc/sysctl.conf:
kernel.sched_child_runs_first = 1
You are right to suggest that due to SMP and due to the general non-determinism of preemption we _never_ made any 'promise' to run the child first.
It was a statistical property based on performance considerations - and now we flipped it around based on latency and for kbuild performance/throughput reasons: Serge Belyshev reported a 7% increase on a quad due to this change and i measured a 1.5% peak-kbuild performance increase.
So it's worth it for multiple reasons and even in the worst-case problems can be worked around easily and without rebooting the system.
Ingo
| |