## A Multithreaded, Transaction-Based Locking Strategy for Containers Bob Steagall CppCon 2020 KEWB COMPUTING ## Overview • Sharing a container among multiple threads • A motivating problem • Some than ever ## Sharing a Container – Avoiding Race Conditions • If all threads are readers... • No locking is required • If the number of reads is much larger than the number of writes... • We might be writes? • A per-element mutex strategy might work... • ... if a given write operation requires locking exactly one element ## Sharing a Container – Avoiding Race Conditions • What about the case where

Copied Value ■ Thread-Specific Storage ■ Future ## Dealing with Mutation ■ Scoped Locking ■ Strategized Locking ■ Thread-Safe Interface ■ Guarded Suspension Active Object Monitor Object Reactor ■ Thread-Specific Storage ■ Future Active Object ## Dealing with Mutation ■ Scoped Locking ■ Strategized Locking ■ Thread-Safe Interface Monitor Object ■ Guarded Suspension Reactor ## Copied with Mutation ■ Scoped Locking ■ Strategized Locking Monitor Object ■ Thread-Safe Interface ■ Guarded Suspension Reactor ## Scoped Locking ## Scoped Locking is RAI applied to locking ## I dea: -

using a libuv threadpool (Experimental) 1126 49.3 Low-level synchronization primitives 1126 50 Scoped Values 1127 50.1 Example 1130 50.2 Idioms 1131 50.3 API docs 1132 50.4 Implementation notes and warnings to be aware of when using threads in Julia: - Base collection types require manual locking if used simultaneously by multiple threads where at least one thread modifies the collection (common via nextidx because they all run on the same process. Even if Tasks are scheduled cooperatively, locking may still be required in some contexts, as in asynchronous I/O. This means context switches only

from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache

with std::lock or s 0.4.6 [14] Do not call virtual functions and c 0.4.7 [15] Avoid deadlock by locking in a pr 0.4.8 [16] Objects of std::lock_guards, std:: 0.4.9 [17] Define a mutex together with the initial revision|straightforward and decidat| |10|0.4.3 \[11] Use std::lock(), std::try\_lock() or std::scoped lock to acquire multiple mutex|required|easy|yes, on local level|accept for initial revision|straightforward later|related API is not yet confir| |16|0.4.11 \[19] There shall be no code path which results in locking of the non-recursive m|mandatory|complex|yes, on system level|accept for initial revision|clear UB

returned? e() { (e.g., equivalent to someone taking the only key for your house, walking in, locking the door, then flushing key down toilet) ed value + 1; thread n (blocked) Critical because we only 3 member functions ## Member functions |(constructor)|constructs a lock\_guard, optionally locking the given mutex (public member function)| |---|---| |(destructor)|destructs the lock\_guard object utex> ## std::scoped_lock - Other mechanisms • std::scoped lock (C++17) - ○ An update to lock_guard, but can acquire multiple locks at once ▪ i.e., std::scoped_lock_scoped_lock(mutex1, mutex2);

library settings Open files list Per-thread last error code Per-archive state ## I mproving PhysFS Locking Split main mutex into several Remove the error code mutex entirely Introduce toggle to disable ## 2 D Assets Bottleneck int CTextureHandler::AddTexture( const string& Filename) { scoped_lock Lock(_Mutex); int Idx = _Textures.Find( Filename ); return Idx != -1 ? Idx : ☐ Load all requested textures ☐ Bind loaded textures to sprites ## Reduce locking scope ## Bottleneck mitigation { scoped_lock Lock(_Mutex); int Idx = _Textures.Find( Filename ); if ( Idx != -1

from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache

from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache

from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache from repodata.jlap; implies 'lock'. lock: use locking when reading, updating index (repodata.json) cache. Now enabled. --no-lock Disable locking when reading, updating index (repodata.json) cache