An interesting question question showed up in my mailbox today. So interesting that I think it’s worth a public answer and discussion:
In chapter 7 of The Art of Unix Programming, you classified threads under the section “Problems and Methods to Avoid”. You also wrote that with the increased emphasis on thread-local storage, threads are looking more like a controlled use of shared memory. This trend has certainly continued; recent programming languages like D, Scala, and Go encourage the use of threads as mostly isolated lightweight processes with message passing. Observing this trend, I have often wondered, why not go all the way and use multiple OS processes? I can think of two reasons to use threads in this newer, controlled way rather than using full processes:
1. Portability to Windows, which doesn’t have an equivalent of fork(2)
2. Performance, particularly because message passing between real processes requires serialization and deserialization, whereas message passing within a process can be done with shared memory and (maybe) locks
So what do you think? Are threads still a menace to be avoided in favor of full OS processes? Or has the situation improved since 2003?
I think it has, and I think you’ve very nearly answered your own question as to why. Bare threads were dangerously prone to deadlocks, livelocks, context-trashing, and various other sorts of synchronization screwups – so language designers set out to encapsulate them in ways that gave better invariants and locality guarantees without sacrificing their performance advantages. I think Scala’s transactional memory stands out as a particularly elegant stab at the problem.
I don’t develop for Windows or communicate much with people who do, so I’m not equipped to judge how important Windows portability is in motivating these features. But the performance issue you called out is real and quite alive on Unix systems.
UPDATE: Matt Campbell, who has materialized in the comments here, send the original question and has given me permission to cite him. Thanks for a good question!