Re: A quota based lock

Eric Sosman <esosman@ieee-dot-org.invalid>
Mon, 08 Aug 2011 07:58:51 -0400
On 8/8/2011 3:13 AM, Robert Stark wrote:

I want to write a lock to control access to a resource, there are
different kind of jobs using this resource, say job A,B,C, at the
beginning, i use Lock api from jdk concurrent package, but i suffered
from serious job starvation, so i want to do something like this:

My idea is input a map of percentages you want to assign for each job,
and provide a simple lock api.

Input A=>20, B=>70, C=>10 means A=>20%, B=>70%, C=>10%
If there's no A jobs pending on the lock, then its quota would be
divided evenly to other pending jobs that is B=>80%, C=>20%.(This rule
apply to other type of jobs as well).

Then i got stuck, the only way i can think about is to introduce an
extra dispatch thread and several queues, can someone give me some

     Your requirements aren't entirely clear to me, but here are a
couple of thoughts:

     If the locks are held for short durations (as most locks should
be), this "quota" notion doesn't seem to make a lot of sense. If a
worker takes the lock, does something critical for a microsecond, then
drops it and goes about its business for a second, whatever problems
you're having aren't really to do with the lock per se. So I guess
you're thinking about a lock that proxies for an unshareable resource
a worker will use for macroscopic time, and it's not truly the lock you
want quotas for, but the resource behind it.

     If so, right away you can see you'll need a way to ensure that a
worker will release the resource "soon" to give other workers a chance
at it. If this property isn't inherent in your design, you'll need to
include some kind of preemption mechanism, a way to wrest the resource
from its holder involuntarily (possibly involving rollbacks or other
kinds of recovery for the worker that got preempted). Lacking either
preemption or a guarantee of timely release, you can't be sure that
Worker A (20% quota) won't hold the resource for six solid months.

     This is the kind of problem an O/S' task scheduler solves: It
doles out various unshareable resources (CPU cycles, RAM locations,
I/O access, ...) to competing consumers. I'm not an expert on the
various techniques used, but I don't think it's a "solved problem"
(if it were, there wouldn't be so many different schedulers). That
is, I don't think there's a one-size-fits-all sure-fire solution to
your problem; you'll need to consider your goals and constraints in
quite a bit of detail to arrive at a tolerable compromise, probably
less than completely satisfactory.

     So, one thought is to give the problem to the O/S' scheduler!
If the unshareable resource is something the O/S understands, maybe
your workers should be separate "tasks" or "jobs" or whatever the
O/S wants to call them. It could save you a lot of hard work if you
can let an existing software component do the job for you.

     Failing that, I'd suggest keeping things as simple as possible.
Start with an ordinary queue, with each worker joining the end of
the queue when it wants the resource and then getting the resource
when it reaches the front. If Worker A holds the resource (and
eventually releases it), all the other workers will get a crack at
it (if they want it) before A gets it again. Don't mess around with
quotas or priorities or whatnot until and unless the simple solution
is found wanting.

     If the simple solution doesn't work out, report back (with more
detail) and I'm sure folks will suggest more sophisticated approaches.
But for starters, KISS.

Eric Sosman

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