On Wed, Jul 18, 2007 at 01:26:48PM +0530, Dhaval Giani wrote: > Hi Andrew, > > I was running kernbench on top of 2.6.22-rc6-mm1 and I got a Hangcheck > alert (This is when kernbench reached make -j). > > Also make -j is hanging.
[refer http://marc.info/?l=linux-kernel&m=118474574807055 for complete report of this bug] Ingo, Dhaval tracked the root cause of this problem to be in cfs (btw cfs patches weren't git-bisect safe). Basically, "make -s -j" workload hanged the machine, leading to lot of OOM killings. This was on a 8-cpu machine with no swap space configured and 4GB RAM. The same workload works "fine" (runs to completion) on 2.6.22. I played with the scheduler tunables a bit and found that the problem goes away if I set sched_granularity_ns to 100ms (default value 32ms). So my theory is this: 32ms preemption granularity is too low value for any compile thread to make "usefull" progress. As a result of this rapid context switch, job retiral rate slows down compared to job arrival rate. This builds up job pressure on the system very quickly (than may have happened with 100ms default granularity_ns or 2.6.22 kernel), leading to OOM killings (and hang). >From a user perspective, who is running with default granularity_ns value, this may be seen as a regression. Perhaps, these new tunables in cfs are something for users to become used to and tune it to approp setting for their system. It would have been nice for kernel to auto-tune the settings based on workload, but I guess that's harder. -- Regards, vatsa - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
