On Wed, Sep 15, 2010 at 12:27 AM, Smital Desai <Smital.Desai@xxxxxxxxxxxxxxx> wrote:
The second task will eventually be context switched out. And at some point the process holding the spinlock will get a chance to run & it will release the spinlock eventually. Having said that, whenever the second process executes, it will keep spinning waiting for the spinlock. Thats why, it is not a good idea to use spinlocks in a uniprocessor environment. Think Correctness Vs. Efficiency. Spinlocks are required to ensure CORRECTNESS in a uniprocessor environment with preemption enabled. However, spinlocks are not the most efficient synchronization primitives that should be used in a uniprocessor environment.
Note that even in multi-processor environments, spinlocks are used in cases where the level of the contention of the lock is usually low. Because here too, the process wastes CPU cycles by waiting for the spinlock. If the contention is high, the wastage will increase.
Regards,
Venkatram Tummala
________________________________________
Thanks and Regards
Smital Desai
From: Tayade, Nilesh [Nilesh.Tayade@xxxxxxxxxxxx]
Sent: Wednesday, September 15, 2010 12:35 PM
To: Smital Desai; Kernel Newbies
Subject: RE: spin locks in uniprocessor system
ok .. i might sound silly but in this case too.. When the second task tries to acquire
> -----Original Message-----
> From: kernelnewbies-bounce@xxxxxxxxxxxx [mailto:kernelnewbies-
> bounce@xxxxxxxxxxxx] On Behalf Of Smital Desai
> Sent: Wednesday, September 15, 2010 11:58 AM
> To: Kernel Newbies
> Subject: spin locks in uniprocessor system
>
> Hi everyone,
>
> It's an extract in LDD3 , regarding spink lock. I have put my query
> in brackets.
>
> Spinlocks are, by their nature, intended for use on multiprocessor
> systems, although a uniprocessor workstation running a preemptive
> kernel behaves like SMP, as far as concurrency is concerned.
> ( I need to understand , how this is true with an example preferably
> )
>>On uniprocessor system even if you have a task scheduled, the interrupt
>>can still come and should be handled by that processor. So let's say you
>>get a timer interrupt - the schedule() will be called on return and if
>>there is any higher priority task waiting, your previous task can get
>>scheduled out.
>>This is referred to as pseudo concurrency (Please refer Robert Love).
the same spin lock , it will again cause the processor to spin continuously hogging it completely ..
then how does the first task gets a chance to execute and release the spin lock.
Note that even in multi-processor environments, spinlocks are used in cases where the level of the contention of the lock is usually low. Because here too, the process wastes CPU cycles by waiting for the spinlock. If the contention is high, the wastage will increase.
Regards,
Venkatram Tummala
>______________________________________________________________________
> If a nonpreemptive uniprocessor system ever went into a spin on a
> lock, it would spin forever; no other thread would ever be able to
> obtain the CPU to release the lock. For this reason, spinlock
> operations on uniprocessor systems without pre-emption enabled are
> optimized to do nothing, with the exception of the ones that change
> the IRQ masking status.
> ( I don't get the meaning of last sentence "with the exception of
> ......" Please can somebody explain )
>
>>I am not sure about this statement, though. Would appreciate if someone
>>can provide some pointers.
>
> Thanks and Regards
> Smital Desai
>
--
Thanks,
Nilesh
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