Lock-free WaitList #113
Lock-free WaitList #113
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# Conflicts: # core/include/userver/engine/impl/wait_list_fwd.hpp # core/src/engine/impl/wait_list_light.cpp # core/src/engine/impl/wait_list_light.hpp # core/src/engine/io/fd_control.cpp # core/src/engine/single_consumer_event.cpp # core/src/engine/task/task_context.cpp # core/src/engine/task/task_context.hpp
# Conflicts: # core/src/engine/impl/wait_list_light.cpp
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| const TaggedPtr<Node> null_node(kNullNode, segment_epoch); | ||
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| // The paper advises randomized iteration order for better performance under | ||
| // contention, but we trade it for better single-threaded performance. |
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This confuses me
My understanding is that the paper is basically "Split everything in chunks of size K, randomize iteration over chunks and hope for the best", but that "randomize and hope" part is missing - don't we just have a lock-free queue that is exactly K times slower for even single-threaded performance?
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Even though the randomization is used mostly to reduce contention, it helps (it does, right?) with this loop not being O(k^2)
As of now it seems to me that we got the worst of both worlds - contention isn't reduced, complexity is O(k^2).
I propose either
- generating some (or even all, with K = 8 there are about 40k of them) possible permutations at compilation and selecting the permutation decently fast somehow or
- doing some bits manipulations here to find a candidate slot in O(1)
First way should reduce contention and might be fast enough anyway, second way should be the fastest one in single-threaded performance
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The same applies to DoTryPopStep
| context->Wakeup(impl::TaskContext::WakeupSource::kWaitList, | ||
| impl::TaskContext::NoEpoch{}); | ||
| void WaitList::WakeupAll() noexcept { | ||
| while (impl_->WakeupOne()) { |
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Random thought: this could potentially be optimized using enqueue_bulk interface of moodycamel
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