[argobots-discuss] constraining overall stack memory allocation

[Shintaro Iwasaki]

Hi Phil,

Thanks for using Argobots! I believe it's about memory consumption issues
regarding ULT stacks.

> What would be ideal for me would be if ABT_thread_create() would defer
stack allocation somehow.
I believe https://github.com/pmodels/argobots/pull/356 (merged) exactly
does this.  This configuration is disabled by default, so please set
--enable-lazy-stack-alloc at configure time.

[Background]
Argobots needs to keep
- "full stacks [*1]" (in this case, 2MB) per "active" (i.e., "executing" +
"suspending") ULT
Intuitively, Argobots must have a full ULT stack to save an intermediate
ULT execution state, in addition to a stack space for a currently executing
ULT.  This is the minimum stack requirement for Argobots.
[*1] There was a long discussion in
https://github.com/pmodels/argobots/issues/274, but basically it's not
possible to allocate small stack first and expand it later within Argobots)

[Ideas]
A ULT stack is assigned when a ULT is executed (not created).  The stack is
reclaimed when a ULT is finished (not freed).  This can achieve the minimum
stack use calculated based on [Background].  See the PR for details.  The
PR explains it using some figures.

[Reduce More]
1. This does not include the ULT stack pool (=cache), so if you want to
further reduce memory usage, please shrink the stack pool size.  This pool
mechanism just increases the constant amount of memory consumption, so this
pool cache won't affect the memory footprint much, I believe.  Shrinking
this can negatively affect the performance.
2. Even if you allocate a stack in this way, still you need 2MB per
"suspended ULT".  If most of the ULTs launch and then immediately yield,
this "enable-lazy-stack-alloc" method does not reduce memory consumption.
If you need to immediately yield, instead of yielding, please create a new
ULT for continuation and exit the ULT; if so, Argobots does not need to
save a full ULT stack per yielded ULT. (A newly created ULT does not have a
ULT stack since it has not started yet).

---

I might not fully understand the use case, but hopefully this flag helps.
Please let me know if you have any questions or suggestions.

Thanks,
Shintaro


On Thu, Jun 16, 2022 at 1:57 PM Phil Carns via discuss <
discuss at lists.argobots.org> wrote:

> Hi all,
>
> I was rummaging around in the code looking for ideas just now and
> figured I might save myself some time by asking on the list to see if
> anyone else has encountered this.
>
> A quick review of the use case: we are using large stack sizes (2 MiB
> right now, though we could probably go lower but it will still be much
> larger than the ABT default).  We also create, execute, and complete a
> large number of detached ULTs.  Only a very few are intentionally long
> lived.
>
> Our current strategy is that a central producer (who drives network
> progress) creates ULTs that may be placed on other pools/ESs depending
> on configuration.
>
> I had *thought* that the ULT stacks were not allocated until the ULT was
> selected for execution by a scheduler, but I see now that's not the
> case.  The stack is allocated up front at ABT_thread_create() time.  I'm
> kicking myself for not understanding that sooner.  It didn't matter so
> much when we used to use small stack sizes.
>
> At any rate, at this point this strategy has a few implications. If the
> ES schedulers don't retire old ULTs fast enough (even if they are very
> "close" to completion) then we can balloon memory consumption even if it
> doesn't look like our actual concurrency is all that high, simply
> because we are greedily taking more memory for stacks without regard to
> ULT completion.  Secondly, the one producer is always paying the
> allocation cost, and the memory is always local to that one core.
>
> What would be ideal for me would be if ABT_thread_create() would defer
> stack allocation somehow.  Ideally not consuming so much memory for a
> thread until a) it can really be executed and b) the scheduler thinks it
> is a good idea to do so.  Even better if the the allocation were in the
> context of the ES that popped the thread, rather than the ES that
> spawned the thread.
>
> Is this possible?
>
> It would be neat if this could be done internal to Argobots somehow for
> generality for my use case, but walking through the code I have the
> sinking feeling that we need to do this above Argobots (explicitly
> queueing up work and letting the "worker" execution streams create their
> own ULTs to perform that work a needed, rather than letting the ULT
> pools within Argobots serve double duty as our work queue).
>
> I'm comfortable with custom pools and schedulers, but it looks like the
> key step is already out of our hands at ULT creation time so there isn't
> much a custom pool or scheduler could do.
>
> Thanks for hearing me out, and thanks in advance for feedback (even if
> it takes the form of "that's a silly idea" :) ).
>
> thanks,
>
> -Phil
>
>
>
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