runtime: eliminate unnecessary lfence while operating on queue::Local<T>
#7340
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I just merged #7339, so merging master should fix the ci issue 👍 |
Darksonn
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| pub(crate) fn remaining_slots(&self) -> usize { | ||
| self.inner.remaining_slots() | ||
| let (steal, _) = unpack(self.inner.head.load(Acquire)); | ||
| // safety: this is the **only** thread that updates this cell. | ||
| let tail = unsafe { self.inner.tail.unsync_load() }; | ||
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| LOCAL_QUEUE_CAPACITY - len(steal, tail) |
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Why not just LOCAL_QUEUE_CAPACITY - self.len()?
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Why not just
LOCAL_QUEUE_CAPACITY - self.len()?
@Darksonn Thanks for your review, I might be mistaken, but I thought the slots that are being stolen are not available for pushing new task.
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Oh I see now. I did not notice that it was using steal instead of head for the first parameter to len.
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Blocked by #7339 which fixes the CI failures caused by the new compilation error message of the new stable toolchain.Motivation
queue::Local<T>is never been send to another thread (except while shutting down), thetailis only updated by the worker thread who owned this local queue, so any change totailis immediately visible to the current thread.This means the
Acquireordering is not required for loadingtailwhile operating on the local queue.Solution
Unsync load
Since the lfence is not required for this scenario, the atomic loading was replaced by simple pointer deref.
Remove all methods of the
queue::Inner<T>Because of the following reasons:
Inner<T>can be implemented in differently way forLocal<T>andSteal<T>len_localorunsync_lencreates possibilities for incorrect usage in the future which causes ABA problems in critical path.I removed all methods on the
Inner<T>and re-implement it for bothLocal<T>andSteal<T>.Remove the feature gate of
Steal<T>::len()The feature gate
cfg_unstable_metricsis removed forSteal<T>::len()to avoid duplicating the same code.This doesn't seem to break anything, but if it does please let me know and I'll switch to something else.
Anything else?
Ideally, the
queue::Local<T>should be marked as!Sendto lower the risk that unintentionally send it to another thread, but it required the changes of the starting and shutdown logic, I will try to open another PR for this improvement.