From 29da090bf5cb2e4cac31f887e619922af04dc6dc Mon Sep 17 00:00:00 2001
From: Gvozden Neskovic <neskovic@compeng.uni-frankfurt.de>
Date: Thu, 6 May 2021 11:04:01 +0200
Subject: [PATCH] use thread local cache to avoid interprocess lock on shm
 GetData

---
 fairmq/shmem/Manager.h | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/fairmq/shmem/Manager.h b/fairmq/shmem/Manager.h
index c6608603..de85ae40 100644
--- a/fairmq/shmem/Manager.h
+++ b/fairmq/shmem/Manager.h
@@ -314,6 +314,7 @@ class Manager
 
                 (fEventCounter->fCount)++;
             }
+            fRegionsGen += 1; // signal TL cache invalidation
             fRegionEventsCV.notify_all();
 
             return result;
@@ -327,8 +328,28 @@ class Manager
 
     Region* GetRegion(const uint16_t id)
     {
+        // NOTE: gcc optimizations. Prevent loading tls addresses many times in the fast path
+        const auto &lTlCache = fTlRegionCache;
+        const auto &lTlCacheVec = lTlCache.fRegionsTLCache;
+        const auto lTlCacheGen = lTlCache.fRegionsTLCacheGen;
+
+        // fast path
+        for (const auto &lRegion : lTlCacheVec) {
+            if ((lRegion.second == id) && (lTlCacheGen == fRegionsGen)) {
+                return lRegion.first;
+            }
+        }
+
         boost::interprocess::scoped_lock<boost::interprocess::named_mutex> lock(fShmMtx);
-        return GetRegionUnsafe(id);
+        // slow path: check invalidation
+        if (lTlCacheGen != fRegionsGen) {
+            fTlRegionCache.fRegionsTLCache.clear();
+        }
+
+        auto *lRegion = GetRegionUnsafe(id);
+        fTlRegionCache.fRegionsTLCache.emplace_back(std::make_pair(lRegion, id));
+        fTlRegionCache.fRegionsTLCacheGen = fRegionsGen;
+        return lRegion;
     }
 
     Region* GetRegionUnsafe(const uint16_t id)
@@ -366,6 +387,7 @@ class Manager
             fShmRegions->at(id).fDestroyed = true;
             (fEventCounter->fCount)++;
         }
+        fRegionsGen += 1; // signal TL cache invalidation
         fRegionEventsCV.notify_all();
     }
 
@@ -613,6 +635,7 @@ class Manager
         using namespace boost::interprocess;
         bool lastRemoved = false;
 
+        fRegionsGen += 1; // signal TL cache invalidation
         UnsubscribeFromRegionEvents();
 
         {
@@ -665,6 +688,12 @@ class Manager
     Uint16SegmentInfoHashMap* fShmSegments;
     Uint16RegionInfoHashMap* fShmRegions;
     std::unordered_map<uint16_t, std::unique_ptr<Region>> fRegions;
+    // make sure this is alone in the cache line: mostly read
+    alignas(128) inline static std::atomic<unsigned long> fRegionsGen = 0ul;
+    inline static thread_local struct ManagerTLCache {
+        unsigned long fRegionsTLCacheGen;
+        std::vector<std::pair<Region*, uint16_t>> fRegionsTLCache;
+    } fTlRegionCache;
 
     std::atomic<bool> fInterrupted;
     std::atomic<int32_t> fMsgCounter; // TODO: find a better lifetime solution instead of the counter