Re: [Boost-users] [Thread] Writer/Reader strange scheduling

Subject: Re: [Boost-users] [Thread] Writer/Reader strange scheduling
From: Stefan Strasser (strasser_at_[hidden])
Date: 2010-08-10 12:03:56

Zitat von Gaetano Mendola <mendola_at_[hidden]>:

> On 08/10/2010 05:05 PM, Roland Bock wrote:
>> On 08/10/2010 04:28 PM, Stefan Strasser wrote:
>>>> isn't that a waste? Why make reader 3 and 4 waiting and then make
>>>> those running
>>>> before the writer? The code to reproduce that is at the end.
>>>
>>> I don't think Boost.Thread specifies this behaviour, so it is probably
>>> platform-dependent.
>>> I'm guessing readers 3 and 4 have to wait because the writer thread
>>> requested an exclusive lock (and otherwise the writer thread would be
>>> starved), but once all locks have been released it is a decision of the
>>> OS scheduler which thread is then woken up first and can acquire a lock
>>> - one of the reader threads in this case apparently.
>>
>> Right, if you want an "order preserving shared mutex", you have to  
>> write it yourself.
>
> That was clear to me, still I think, I don't know who fault is,  
> having a reader
> waiting and then being scheduled is a waste :D
>
> I guess I'll write one my self.

don't you need OS support for this kind of scheduling?
I guess you can write a mutex in userspace that only "preserves  
order", but as long as you still want threads to be scheduled based on  
workload/time slices Í think you need an OS scheduler supporting  
shared mutexes to improve this situation.