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0 pointslukesandberg14y ago0 comments

Luckily these days people are designing algorithms/data structures that don't rely on DCAS :) So you can actually implement them on real hardware

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2 comments · 1 top-level

marshray14y ago· 1 in thread

Hmm, maybe they should.

In the paper you linked they don't seem to be able to perform a priority queue op in under about 1 us. Even when there are only 15 threads running on an exotic 29-core machine, they can only handle 750e3 ops/second (with no workload on each op). (This is consistent with a benchmark I did on Boost.ASIO a while back.

So the tasks should be sized to take at least (thread cnt)* 9 us to complete to keep the overhead from the priority queue overhead under about 10%. The best cases for the lock free algorithm might let you bring that down to about 3 us in theory. I'm not sure that justifies the additional complexity.

This survey paper http://www.zurich.ibm.com/pdf/sys/adv_messaging/shortConcurr... says a lot of stuff and then at the end Figure 4 shows a simple coarse-locked single-threaded structure providing double the performance when there is actual load involved and significant preemption. Which is the opposite of what I'd expected, which would be the occasional preemption of the thread holding the coarse lock would cause a slowdown.

lukesandbergOP14y ago

In my investigations i did find the overhead to be fairly high and in most of my benchmarks i found that using simple fixed size fifo queues was significantly faster. However my benchmarks were generally simple cpu bound tasks you would definitely want to explore more varied workloads. Also since you are using a priority based scheduling algorithm presumably the priorities are significant and therefore sacrificing some throughput would be worth it.

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