[argobots-discuss] question about ABT_mutex and ULT scheduling

[Iwasaki, Shintaro]

Hi Phil,

Thank you for a good question! I created an issue: https://github.com/pmodels/argobots/issues/287

Yes, what you expect is correct.  A ULT (lock/unlock) may not yield if there is no contention.  We guarantee this and will make it clear in the specification.

The current Argobots (assuming the current master) should work as you expect; ULT A should never yield in your case.

In the case of Argobots 1.0 or 1.0.1, a ULT may yield because of the following possible reasons, both of which are fixed in the current master:
1. Lock is not performed atomically "strong" while the architecture supports weak atomics (e.g., on ARM and POWER) (fixed by https://github.com/pmodels/argobots/pull/223)
2. If you explicitly pass `--disable-simple-mutex` at configuration time, the previous mutex-handover mechanism may have this issue (fixed by https://github.com/pmodels/argobots/pull/268)

Regarding 1., because some atomic instructions spuriously fail ("weak" https://en.cppreference.com/w/c/atomic/atomic_compare_exchange), maybe the current spinlock implementation in Argobots causes this issue if you are using non-Intel hardware.  I'd be happy if you could let us know what combination of hardware and compiler (with a compiler version) you are using.  If you are using Intel hardware, I believe the current Argobots master work correctly unless you use a not-so-common compiler (e.g., PGI), but I will check.

(Note that a priority lock/unlock is just a hint, so it will not help.)

Anyway, I should make this point clearer in the specification.  At the same time, I will add a test to see if this is really the case.  If the current mechanism is broken, I will fix it.  Please estimate that this clarification and fix (if possible) will come this week.

Thanks,
Shintaro Iwasaki

________________________________
From: Phil Carns via discuss <discuss at lists.argobots.org>
Sent: Wednesday, January 6, 2021 12:27 PM
To: discuss at lists.argobots.org <discuss at lists.argobots.org>
Cc: Carns, Philip H. <carns at mcs.anl.gov>
Subject: [argobots-discuss] question about ABT_mutex and ULT scheduling

Hi all,

We've isolated a situation where the ABT_mutex construct is behaving a
little differently than I expected.  We have two ULTs running on a
single ES.  The ULTs are using ABT_mutex_lock/free() to protect a shared
data structure.  This specific configuration will never have lock
contention (the mutex is really there to protect more complex
configurations where there are more ESs and ULTs participating than what
I described above).

Here is what puzzles me: I'm not 100% sure, but it really looks like ULT
A yields to ULT B when attempting to lock the mutex sometimes, even
though there is no contention.

This is a performance bug for us; ULT B is only supposed to execute when
ULT A is idle in this configuration.  We don't really want to give up
execution when acquiring an uncontested mutex if we don't have to.

I'm sure we could work around it (restructuring our code, or using a
spinlock or priority mutex or something), but I wanted to ask on the
list first: is the behavior I described above (a ULT yielding on a mutex
lock, even if the mutex is available) expected?  Or is it an indication
that we are doing something wrong somewhere?  I want to make sure that I
understand the problem before altering the code.

thanks!

-Phil


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