Re: [PATCH v0] add nano semaphore in kernel

[Rakib Mullick]

On Sun, Dec 26, 2010 at 6:05 PM, Hillf Danton <dhillf@xxxxxxxxx> wrote:
> On Sun, Dec 26, 2010 at 5:08 PM, Rakib Mullick <rakib.mullick@xxxxxxxxx> wrote:
>> On Sun, Dec 26, 2010 at 1:04 PM, Hillf Danton <dhillf@xxxxxxxxx> wrote:
>>> On Sun, Dec 26, 2010 at 2:46 PM, Rakib Mullick <rakib.mullick@xxxxxxxxx> wrote:
>>>> On Sun, Dec 26, 2010 at 11:13 AM, Hillf Danton <dhillf@xxxxxxxxx> wrote:
>>>> Above description tells how its done, what it is but its not clear why
>>>> we should use it. Can I know why should we use it or its benefits?
>>>>
>>>
>>> The outstanding benefit looks that nano semaphore could be used in
>>> cases where callers want to wait not only more than one jiffy, but far
>>> less than one jiffy also.
>>>
>> But, how do we know that resources are going to be available within
>> one jiffy or far less than one jiffy? We we be deterministic?
>>
> This is a really hard question.
>
> Though I could not answer as fine as Ingo Molnar could, the
> deterministic has to be faced by most contentions for resources in
> kernel, simply because deadlock could occur in spin lock for instance.
>
Yes, if its not handled carefully.

> On the other hand, this question explores a byproduct advantage, not
> seriously considered before,  of nano semaphore, that the caller could
> learn that there is something out of track if she waited over 100
> microseconds in 5 consequent steps, 20 microseconds a step, if she
> think the resource should be available in 60 microseconds.
>
Why should caller think such a way, that resource will be available in
60 microseconds?

> And in nano semaphore method is available for callers to select
> waiting over one second, one millisecond, one microsecond, one
> nanosecond, depending on what the underlying system could offer. After
> waiting, the caller is free to determine what to do next if the
> resource is not available.
>
How do we know that, what the underlying system will offer? Its an
NP-type problem. We cannot determine what will happen on a systems
context. When a caller need resource, without that resource is it
possible to accomplish its job? If its not, then the ultimate way to
deal with it, is simply waiting until it gets the resource.

> Here is another sample, if taxi will not come in two minutes, I could
> either give up shopping downtown, or wait another three minutes,
> depending on what will happen two minutes later.

But what, if your mother told you not to come home until you done shopping? :)

thanks,
rakib

>
> thanks
> Hillf
>
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