Re: Am I using NSConditionLock correctly?
Re: Am I using NSConditionLock correctly?
- Subject: Re: Am I using NSConditionLock correctly?
- From: jkp <email@hidden>
- Date: Tue, 11 Oct 2005 09:04:02 +0100
On 10 Oct 2005, at 22:57, Steve Weller wrote:
On Oct 10, 2005, at 1:37 PM, jkp wrote:
The way you have described it, you are making your main thread
sleep on the condition lock whilst your worker does the
work...blocking in the process. the simple question i ask you
is...what does this buy you?
My main thread has to wait for the worker thread to get into a
known state so I can save my objects to disk. So I need to block my
main thread until the worker thread says "I've stopped now". It is
a short amount of time -- short enough that the interface will not
suffer during the block.
You might as well just do the calculation on the main thread and
simplify your life! If you have the main thread sleep on the lock
whilst the worker does its work you are still blocking, so you
achieve the same thing.
The main thread keeps the GUI alive, so it cannot sleep under
normal circumstances. And this whole project is an exercise in
using a computationally-intensive worker thread.
surely if you want to keep the GUI "alive" it needs to be able to
respond no? It will not respond if you have it sitting on a lock. I
understand you want to save data when it is in a known good state -
so why not engineer things so the rest of your program doesnt touch
that data whilst the worker is busy with it, and then the worker can
signal to you when it is done using one of the methods i suggested.
I recently implemented worker class to use throughout my current
application and i learnt a lot in the process. A condition lock
is a great way to go since it allows you to have your workers
sleep efficiently and for you to exact granular control on them
when you need their services.
The way you describe is interesting. I am currently starting and
stopping my worker thread by creating and destroying it. But I have
reasons for doing that: I need to be able to throw away its state
and give it a new state to continue calculating with. It's easier
to throw away the whole thread and start a new one. I can see how
condition locks can be used to signal a thread what to do and read
its state back.
I took this a step further actually an implemented a custom queue (in
the form of a CFArray). A set of predefined instructions are passed
via the queue to the worker, and it runs its own special loop to
process the instructions. On each iteration it checks the lock (and
can also check at other points during the threaded routines)....the
lock tells the worker if it should continue worker, stop, start work
or die etc....the Queue itself holds the details of what is to be
done (an invocation is the most common content of the queue).
I think you could do with defining a set of conditions that might
apply to your thread at any point during its life cycle. Maybe
something like NOWORK, WORKPENDING, DATAPENDING etc...You can use
these to notify the worker what it needs to do. The worker can do
its thing and when it is done you have a choice of ways to send
data / notify the main thread that things are finished. My
favorite is NSObject's
performSelectorOnMainThread:withObject:waitUntilDone: method. You
can use this to call a routine on the main thread and if you want,
wait until it is finished before your worker continues on its
way. This way your main thread can carry on accepting user input
etc, but each time it returns to its runloop it gets a chance to
process a pending request from the worker to perform a selector.
I use that to update my GUI: progress bars etc. and to avoid having
to lock umpteen things. But I am actually finding the synchronous
nature of the call limiting. Not only that but async calls to main
thread methods are secretly synced up somewhere to, so thwarting
some other things I have tried.
If you mean they dont arrive as you expect them to, that is because
they are arriving on an NSPort of some kind. This as you probably
know is another way of communicating between threads which is nice in
some ways, but it has its limitations - namely that the queue of
instructions that can arrive on the thread cannot be controlled with
any precision - it is of a fixed length so will reject instructions
after a certain point, and also if you have more than one port you
cannot predict the order that messages will arrive in. My second
iteration had used ports exclusively, but i found them too hard to
control.
Another way you could do it would be to use the condition lock to
signal you have data, and maybe use a timer to check the lock from
the main thread - again, this way you dont lock the main thread,
but when the timer fires and there is data, the main thread does
the right thing. I dont like this method myself since it is
basically polling, but that would be one way you could use the
condition lock in this way if you wanted to.
Yes, I am desperately trying to avoid poll/sleep, but may have to
go that way if all else fails.
These are just some thoughts and ideas, but i think you could do
with working out exactly what you want your worker thread
implementation to be capable of, and then perhaps write a small
test app that proves that it can do these things as you want. I'd
also take time to stress test the classes you write since you will
be suprised what problems can arrise that you hadnt thought of.
I'm guilty of lack of design forethought and experimentation. But
with good reason: I have no thoughts to put in the fore, since I am
learning this as I go along, and for experimentation: this is it.
If its any comfort it took me 3 iterations to get a pair of
classes that do the job for me and that i am happy with.
Multithreading is a tough topic so dont be suprised if you need to
go back and rehash your design several times before you get
something that works for you.
I'm on number three right now I think. What surprises me is that
multithreading is much harder than multiprocessing and/or reentrant
interrupt-driven code. It's slippery, but not entirely so.
My next attempt is to do exactly what I have been doing but with a
global (static) lock. This should circumvent all the swizzling and
whatever is syncing for me I hope.
Im not sure if this is the way to go. A condition lock is much more
useful because you can convey state as well as availibility. Im
still not clear on why you think it is worth offloading work to the
worker thread whilst leaving the main thread blocking - the interface
is not going to "stay alive" instead it will freeze which is surely
not what you want.
Multithreading is indeed a darn hard exercise, since it is almost
like returning to the bad old days of an unprotected shared memory
space used by mutliple processes (in this case threads). to say that
keeping everything valid and in sync is an art would be an
understatement!
Jamie
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