Re: [PATCH 4/5] x86,smp: keep spinlock delay values per hashed spinlock address

[Rik van Riel <riel@redhat.com>]

On 01/10/2013 08:01 AM, Michel Lespinasse wrote:
> On Tue, Jan 8, 2013 at 2:31 PM, Rik van Riel <riel@redhat.com> wrote:
>> From: Eric Dumazet <eric.dumazet@gmail.com>
>>
>> Eric Dumazet found a regression with the first version of the spinlock
>> backoff code, in a workload where multiple spinlocks were contended,
>> each having a different wait time.
>>
>> This patch has multiple delay values per cpu, indexed on a hash
>> of the lock address, to avoid that problem.
>>
>> Eric Dumazet wrote:
>>
>> I did some tests with your patches with following configuration :
>>
>> tc qdisc add dev eth0 root htb r2q 1000 default 3
>> (to force a contention on qdisc lock, even with a multi queue net
>> device)
>>
>> and 24 concurrent "netperf -t UDP_STREAM -H other_machine -- -m 128"
>>
>> Machine : 2 Intel(R) Xeon(R) CPU X5660  @ 2.80GHz
>> (24 threads), and a fast NIC (10Gbps)
>>
>> Resulting in a 13 % regression (676 Mbits -> 595 Mbits)
>>
>> In this workload we have at least two contended spinlocks, with
>> different delays. (spinlocks are not held for the same duration)
>>
>> It clearly defeats your assumption of a single per cpu delay being OK :
>> Some cpus are spinning too long while the lock was released.
>>
>> We might try to use a hash on lock address, and an array of 16 different
>> delays so that different spinlocks have a chance of not sharing the same
>> delay.
>>
>> With following patch, I get 982 Mbits/s with same bench, so an increase
>> of 45 % instead of a 13 % regression.
>
> Note that these results were with your v1 proposal. With v3 proposal,
> on a slightly different machine (2 socket sandybridge) with a similar
> NIC, I am not seeing the regression when not using the hash table. I
> think this is because v3 got more conservative about mixed spinlock
> hold times, and converges towards the shortest of the hold times in
> that case.

Eric,

with just patches 1-3, can you still reproduce the
regression on your system?

In other words, could we get away with dropping the
complexity of patch 4, or do we still need it?


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