openstack team mailing list archive

[Yun Mao <yunmao@xxxxxxxxx>]

Hi Phil, I'm a little confused. To what extend does sleep(0) help?

It only gives the greenlet scheduler a chance to switch to another
green thread. If we are having a CPU bound issue, sleep(0) won't give
us access to any more CPU cores. So the total time to finish should be
the same no matter what. It may improve the fairness among different
green threads but shouldn't help the throughput. I think the only
apparent gain to me is situation such that there is 1 green thread
with long CPU time and many other green threads with small CPU time.
The total finish time will be the same with or without sleep(0), but
with sleep in the first threads, the others should be much more
responsive.

However, it's unclear to me which part of Nova is very CPU intensive.
It seems that most work here is IO bound, including the snapshot. Do
we have other blocking calls besides mysql access? I feel like I'm
missing something but couldn't figure out what.

Thanks,

Yun


On Fri, Mar 2, 2012 at 2:08 PM, Day, Phil <philip.day@xxxxxx> wrote:
> I didn't say it was pretty - Given the choice I'd much rather have a threading model that really did concurrency and pre-emption all the right places, and it would be really cool if something managed the threads that were started so that is a second conflicting request was received it did some proper tidy up or blocking rather than just leaving the race condition to work itself out (then we wouldn't have to try and control it by checking vm_state).
>
> However ...   In the current code base where we only have user space based eventlets, with no pre-emption, and some activities that need to be prioritised then forcing pre-emption with a sleep(0) seems a pretty small bit of untidy.   And it works now without a major code refactor.
>
> Always open to other approaches ...
>
> Phil
>
>
> -----Original Message-----
> From: openstack-bounces+philip.day=hp.com@xxxxxxxxxxxxxxxxxxx [mailto:openstack-bounces+philip.day=hp.com@xxxxxxxxxxxxxxxxxxx] On Behalf Of Chris Behrens
> Sent: 02 March 2012 19:00
> To: Joshua Harlow
> Cc: openstack; Chris Behrens
> Subject: Re: [Openstack] eventlet weirdness
>
> It's not just you
>
>
> On Mar 2, 2012, at 10:35 AM, Joshua Harlow wrote:
>
>> Does anyone else feel that the following seems really "dirty", or is it just me.
>>
>> "adding a few sleep(0) calls in various places in the Nova codebase
>> (as was recently added in the _sync_power_states() periodic task) is
>> an easy and simple win with pretty much no ill side-effects. :)"
>>
>> Dirty in that it feels like there is something wrong from a design point of view.
>> Sprinkling "sleep(0)" seems like its a band-aid on a larger problem imho.
>> But that's just my gut feeling.
>>
>> :-(
>>
>> On 3/2/12 8:26 AM, "Armando Migliaccio" <Armando.Migliaccio@xxxxxxxxxxxxx> wrote:
>>
>> I knew you'd say that :P
>>
>> There you go: https://bugs.launchpad.net/nova/+bug/944145
>>
>> Cheers,
>> Armando
>>
>> > -----Original Message-----
>> > From: Jay Pipes [mailto:jaypipes@xxxxxxxxx]
>> > Sent: 02 March 2012 16:22
>> > To: Armando Migliaccio
>> > Cc: openstack@xxxxxxxxxxxxxxxxxxx
>> > Subject: Re: [Openstack] eventlet weirdness
>> >
>> > On 03/02/2012 10:52 AM, Armando Migliaccio wrote:
>> > > I'd be cautious to say that no ill side-effects were introduced. I
>> > > found a
>> > race condition right in the middle of sync_power_states, which I
>> > assume was exposed by "breaking" the task deliberately.
>> >
>> > Such a party-pooper! ;)
>> >
>> > Got a link to the bug report for me?
>> >
>> > Thanks!
>> > -jay
>>
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