Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

From: Raghavendra K T
Date: Mon Jun 03 2013 - 02:17:33 EST

On 06/03/2013 07:10 AM, Raghavendra K T wrote:
On 06/02/2013 09:50 PM, Jiannan Ouyang wrote:
On Sun, Jun 2, 2013 at 1:07 AM, Gleb Natapov <gleb@xxxxxxxxxx> wrote:

High level question here. We have a big hope for "Preemptable Ticket
Spinlock" patch series by Jiannan Ouyang to solve most, if not all,
ticketing spinlocks in overcommit scenarios problem without need for PV.
So how this patch series compares with his patches on PLE enabled

No experiment results yet.

An error is reported on a 20 core VM. I'm during an internship
relocation, and will start work on it next week.

Preemptable spinlocks' testing update:
I hit the same softlockup problem while testing on 32 core machine with
32 guest vcpus that Andrew had reported.

After that i started tuning TIMEOUT_UNIT, and when I went till (1<<8),
things seemed to be manageable for undercommit cases.
But I still see degradation for undercommit w.r.t baseline itself on 32
core machine (after tuning).

(37.5% degradation w.r.t base line).
I can give the full report after the all tests complete.

For over-commit cases, I again started hitting softlockups (and
degradation is worse). But as I said in the preemptable thread, the
concept of preemptable locks looks promising (though I am still not a
fan of embedded TIMEOUT mechanism)

Here is my opinion of TODOs for preemptable locks to make it better ( I
think I need to paste in the preemptable thread also)

1. Current TIMEOUT UNIT seem to be on higher side and also it does not
scale well with large guests and also overcommit. we need to have a
sort of adaptive mechanism and better is sort of different TIMEOUT_UNITS
for different types of lock too. The hashing mechanism that was used in
Rik's spinlock backoff series fits better probably.

2. I do not think TIMEOUT_UNIT itself would work great when we have a
big queue (for large guests / overcommits) for lock.
one way is to add a PV hook that does yield hypercall immediately for
the waiters above some THRESHOLD so that they don't burn the CPU.
( I can do POC to check if that idea works in improving situation
at some later point of time)

Preemptable-lock results from my run with 2^8 TIMEOUT:

ebizzy (records/sec) higher is better
base stdev patched stdev %improvement
1x 5574.9000 237.4997 3484.2000 113.4449 -37.50202
2x 2741.5000 561.3090 351.5000 140.5420 -87.17855
3x 2146.2500 216.7718 194.8333 85.0303 -90.92215
4x 1663.0000 141.9235 101.0000 57.7853 -93.92664
dbench (Throughput) higher is better
base stdev patched stdev %improvement
1x 14111.5600 754.4525 3930.1602 2547.2369 -72.14936
2x 2481.6270 71.2665 181.1816 89.5368 -92.69908
3x 1510.2483 31.8634 104.7243 53.2470 -93.06576
4x 1029.4875 16.9166 72.3738 38.2432 -92.96992

Note we can not trust on overcommit results because of softlock-ups

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