Messages in this thread |  | | | From | Nikhil Rao <> | | Date | Thu, 28 Apr 2011 11:20:36 -0700 | | Subject | Re: [RFC][PATCH 00/18] Increase resolution of load weights |
| |
On Thu, Apr 28, 2011 at 4:48 AM, Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com> wrote: > On Thu, 28 Apr 2011 12:37:27 +0530, "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com> wrote: >> On Wed, 20 Apr 2011 13:51:19 -0700, Nikhil Rao <ncrao@google.com> wrote: >> > Hi All, >> > >> > I have attached an early version of a RFC patchset to increase resolution of >> > sched entity load weights. This RFC introduces SCHED_LOAD_RESOLUTION which >> > scales NICE_0_LOAD by a factor of 1024. The scaling is done internally and should >> > be completely invisible to the user. >> > >> > Why do we need this? >> > This extra resolution allows us to scale on two dimensions - number of cpus and >> > the depth of hierarchies. It also allows for proper load balancing of low weight >> > task groups (for eg., nice+19 on autogroup). >> > >> > One of the big roadblocks for increasing resolution is the use of unsigned long >> > for load.weight, which on 32-bit architectures can overflow with ~48 max-weight >> > sched entities. In this RFC we convert all uses of load.weight to u64. This is >> > still a work-in-progress and I have listed some of the issues I am still >> > investigating below. >> > >> > I would like to get some feedback on the direction of this patchset. Please let >> > me know if there are alternative ways of doing this, and I'll be happy to >> > explore them as well. >> > >> > The patchset applies cleanly to v2.6.39-rc4. It compiles for i386 and boots on >> > x86_64. Beyond the basic checks, it has not been well tested yet. >> > >> > Major TODOs: >> > - Detect overflow in update shares calculations (time * load), and set load_avg >> > to maximum possible value (~0ULL). >> > - tg->task_weight uses an atomic which needs to be updates to 64-bit on 32-bit >> > machines. Might need to add a lock to protect this instead of atomic ops. >> > - Check wake-affine math and effective load calculations for overflows. >> > - Needs more testing and need to ensure fairness/balancing is not broken. >> > >> Hi Nikhil, >> >> I did a quick test for creating 600 cpu hog tasks with and without this >> patches on a 16cpu machine(x86_64) and I am seeing some mis-behaviour: >> >> Base kernel - 2.6.39-rc4 >> >> [root@krm1 ~]# time -p ./test >> real 43.54 >> user 0.12 >> sys 1.05 >> [root@krm1 ~]# >> >> Base + patches >> >> [root@krm1 ~]# time -p ./test >> >> Takes almost infinity, after 2 minutes I see only 16 tasks created >> viewed from another ssh session to the machine: >> > I could get this working using following patch, not sure if it has other > implications though. With this, I am back to saner time values for > creating 600 cpu hog tasks: > > [root@ ~]# time -p ./test > real 45.02 > user 0.13 > sys 1.07 > [root@ ~]# >
Nikunj,
Thanks for running the tests and identifying this issue. You are right -- we need to scale the reference weight, else we end up with slices that are 2^10 times the expected value. Thanks for the patch.
-Thanks, Nikhil
> =================================================================== > From: Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com> > > sched: calc_delta_mine - fix calculation > > All the calculations of inv_weight takes scaled down weight, while > calculating the tmp, weight is not scaled down by > SCHED_LOAD_RESOLUTION, which then will return big values because of > which the sched_slice thinks that its not time to preempt the > current running task > > Signed-off-by: Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com> > > Index: kernel/sched.c > =================================================================== > --- kernel/sched.c.orig 2011-04-28 16:34:24.000000000 +0530 > +++ kernel/sched.c 2011-04-28 16:36:29.000000000 +0530 > @@ -1336,7 +1336,7 @@ calc_delta_mine(unsigned long delta_exec > lw->inv_weight = 1 + (WMULT_CONST - w/2) / (w + 1); > } > > - tmp = (u64)delta_exec * weight; > + tmp = (u64)delta_exec * (weight >> SCHED_LOAD_RESOLUTION); > /* > * Check whether we'd overflow the 64-bit multiplication: > */ > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |