Single-threading not considered harmful [LWN.net]

Single-threading not considered harmful

Posted May 20, 2011 19:48 UTC (Fri) by mikernet (guest, #75071)
In reply to: Single-threading not considered harmful by quotemstr
Parent article: Scale Fail (part 1)

You can achieve both multi-threading and multi-process/multi-machine scalability, and should for a large application. A single thread serving many concurrent users will be very latent. Spinning up a seperate process for each user is incredibly resource heavy, processes are much heavier than threads.

Your application should be multi-threaded so that all the processor cores and idle time on any one machine are fully utilized. It should also be able to scale across machines so you can increase total processing capacity easily.

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Single-threading not considered harmful

Posted May 20, 2011 22:44 UTC (Fri) by raven667 (subscriber, #5198) [Link]

I don't have any test data in front of me but I'm not sure your statement is entirely true. On Linux threads are not significantly lighter than processes because processes are extremely light weight. Both are scheduled the same in the kernel and with copy on write memory management there isn't that much difference for the in-kernel housekeeping between creating a thread or forking (cloning) a process.

On a modern multi-socket multi-core machine each socket is its own largely independent computer and the whole machine is a NUMA cluster. That means that each process is assigned to a particular node and that's where its memory lives, splitting memory between nodes or bouncing a process between different nodes reduces performance. My guess is that threading will scale weirdly when you get beyond what can be handled by all the cores in one socket whereas a multi-process model can keep more memory local to the socket the process is running on.

I would suggest starting with a multi-process model because you get better fault tolerance with memory protection then consider threading if that doesn't test out for concurrent performance