From 1055a7aa2ea4296b476cb55099bb7d42e24c74b3 Mon Sep 17 00:00:00 2001
From: Florian Fischer <florian.fischer@muhq.space>
Date: Mon, 26 Jul 2021 10:44:22 +0200
Subject: [PATCH] [IoContext] wrap CQ locking in if constexpr
We don't need to pay the overhead of the atomic operations on each
dispatch loop if there is no concurrent access to worker CQs.
---
emper/io/IoContext.cpp | 43 +++++++++++++++++++++++-------------------
emper/io/IoContext.hpp | 2 ++
2 files changed, 26 insertions(+), 19 deletions(-)
diff --git a/emper/io/IoContext.cpp b/emper/io/IoContext.cpp
index b34c5e12..83ab7001 100644
--- a/emper/io/IoContext.cpp
+++ b/emper/io/IoContext.cpp
@@ -211,24 +211,26 @@ reap_cqes:
LOGD("Reaping completions for worker " << std::to_string(worker->getWorkerId()));
std::array<struct io_uring_cqe *, CQE_BATCH_COUNT> cqes;
- // Someone else is currently reaping completions
- if constexpr (callerEnvironment == CallerEnvironment::EMPER) {
- if (unlikely(!cq_lock.try_lock())) {
- LOGD("worker unsuccessful try_lock");
- return 0;
- }
- } else {
- // We can not reap completions of this IoContext to not race
- // with the sleeping worker.
- if (waitInflight.load(std::memory_order_acquire)) {
- LOGD("Not reaping worker " << std::to_string(worker->getWorkerId())
- << " since this worker is already waiting for its CQEs");
- return 0;
- }
+ if constexpr (needsCqLock) {
+ // Someone else is currently reaping completions
+ if constexpr (callerEnvironment == CallerEnvironment::EMPER) {
+ if (unlikely(!cq_lock.try_lock())) {
+ LOGD("worker unsuccessful try_lock");
+ return 0;
+ }
+ } else {
+ // We can not reap completions of this IoContext to not race
+ // with the sleeping worker.
+ if (waitInflight.load(std::memory_order_acquire)) {
+ LOGD("Not reaping worker " << std::to_string(worker->getWorkerId())
+ << " since this worker is already waiting for its CQEs");
+ return 0;
+ }
- if (!cq_lock.try_lock_or_increment()) {
- LOGD("Global completer unsuccessful try_lock_or_increment");
- return 0;
+ if (!cq_lock.try_lock_or_increment()) {
+ LOGD("Global completer unsuccessful try_lock_or_increment");
+ return 0;
+ }
}
}
@@ -245,7 +247,10 @@ reap_cqes:
io_uring_cq_advance(&ring, count);
- uint32_t globalCompleterAttempts = cq_lock.unlock();
+ uint32_t globalCompleterAttempts;
+ if constexpr (needsCqLock) {
+ globalCompleterAttempts = cq_lock.unlock();
+ }
LOGD("got " << count << " cqes from worker " << worker->getWorkerId() << "'s io_uring");
@@ -336,7 +341,7 @@ reap_cqes:
}
// check if lost wakeup was possible
- if constexpr (callerEnvironment == CallerEnvironment::EMPER) {
+ if constexpr (needsCqLock && callerEnvironment == CallerEnvironment::EMPER) {
bool reReap = false;
// TODO: How sure are we that this is unlikely?
if (unlikely(globalCompleterAttempts > maxRaceFreeCompleterAttempts)) {
diff --git a/emper/io/IoContext.hpp b/emper/io/IoContext.hpp
index 841246ec..f849bb6f 100644
--- a/emper/io/IoContext.hpp
+++ b/emper/io/IoContext.hpp
@@ -68,6 +68,8 @@ class IoContext : public Logger<LogSubsystem::IO> {
Runtime &runtime;
static thread_local IoContext *workerIo;
+ static constexpr bool needsCqLock =
+ emper::IO_COMPLETER_BEHAVIOR != emper::IoCompleterBehavior::none;
// TryLock protecting the completion queue of ring.
ALIGN_TO_CACHE_LINE CqLock cq_lock;
struct io_uring ring;
--
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