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



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
processors?


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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