https://cs.lozic.me/areas/concurrency/Recent content in Concurrency on CS TheoremsHugoen-usFri, 24 Dec 2027 12:00:00 +0100https://cs.lozic.me/posts/t052-mutual-exclusion-without-hardware-support/Fri, 24 Dec 2027 12:00:00 +0100https://cs.lozic.me/posts/t052-mutual-exclusion-without-hardware-support/<h2 id="symptom">Symptom</h2> <p>Two threads increment a shared counter a million times each. The final value is 1,374,522.</p> <p>You know why: the increment is a read, an add, and a write, and the interleavings lose updates. The fix is a lock. But a lock is provided by your runtime, which gets it from the OS, which gets it from a hardware instruction — <code>lock xchg</code>, <code>cmpxchg</code>, <code>ldrex</code>/<code>strex</code>.</p>https://cs.lozic.me/posts/t109-work-stealing-and-fair-scheduling/Fri, 17 Dec 2027 12:00:00 +0100https://cs.lozic.me/posts/t109-work-stealing-and-fair-scheduling/<h2 id="symptom">Symptom</h2> <p>You parallelize a recursive algorithm. Quicksort, or a tree traversal, or a divide-and-conquer numeric kernel. You have 16 cores and a central task queue, and you measure a speedup of 3.</p>https://cs.lozic.me/posts/t049-model-checking-ltl-and-buchi-automata/Fri, 26 Nov 2027 12:00:00 +0100https://cs.lozic.me/posts/t049-model-checking-ltl-and-buchi-automata/<h2 id="symptom">Symptom</h2> <p>Your distributed lock service deadlocks once a month. You cannot reproduce it.</p> <p>The logs show three nodes in states that, as far as you can reconstruct, should not co-occur. You add tracing. It does not happen again for six weeks, then happens twice in a day during a deploy. You write a stress test that runs a million random interleavings overnight and finds nothing.</p>https://cs.lozic.me/posts/t051-the-coffman-conditions/Fri, 29 Jan 2027 12:00:00 +0100https://cs.lozic.me/posts/t051-the-coffman-conditions/<h2 id="symptom">Symptom</h2> <p>Production is wedged. Not slow — wedged. CPU is at zero, no errors are being logged, and two threads are sitting in a lock wait with no timeout. The stack traces show thread A holding the account lock and waiting for the ledger lock, thread B holding the ledger lock and waiting for the account lock. Somebody restarts the service and it goes away.</p>https://cs.lozic.me/posts/t081-amdahls-law/Fri, 18 Sep 2026 12:00:00 +0200https://cs.lozic.me/posts/t081-amdahls-law/<h2 id="symptom">Symptom</h2> <p>The profile was unambiguous: 94% of wall-clock time in one loop, and the loop&rsquo;s iterations are independent. You parallelise it across sixteen cores, the flame graph flattens exactly as predicted, and the end-to-end time goes from 40 seconds to 17.</p>