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@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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