CODEWITHSUNDEEP
cmultithreadingthread-safetypthreads
slartibartfast
slartibartfast

Reputation: 4428

How often does processor cache flush?

Say I have a casual single-byte variable. I think on pretty much all systems single-byte operations are atomic, but if not please let me know. Now, say one thread updates this variable. How long should I expect/prepare for this update to appear in the other threads? I know I can put the update around mutexes/locks/barriers to make sure it's synchronized everywhere, but I'm curious about this. The wait time probably varies depending on whether the other threads are on separate processors/cores, and maybe depending on processor type.

Am I being logical for wondering this or have I greatly misunderstood something?

Upvotes: 4

Views: 607

Answers (3)

ankur
ankur

Reputation: 74

On MIPS architecture there is a sync instruction which serves as a load store barrier across cores i.e all loads and stores before issuing sync will happen before any load and stores after sync.Not sure about if there is an equivalent instruction in x86(assuming that is the architecture you are using).

Upvotes: 0

Martin Thompson
Martin Thompson

Reputation: 16792

In many architectures, the processor won't flush the cache until it has to - to make way for some more-needed data.

However, if the threads are sharing memory space, and you only have a single core, they will be able to see the update "immediately" from the cache. If it's actually been written from the CPU to memory. Which it may not be if the compiler's decided to keep it in a register, in which case your threads will all have their own "local" and incorrect copy.

As others have said, it's an interesting question - but the right answer for synchronising is to use proper synchronisation primitives!

Upvotes: 3

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215193

Memory is synchronized as soon as you call a synchronization primitive/memory barrier such as pthread_mutex_lock. Aside from that, you should not assume any synchronization unless you're using C11 atomic types.

Upvotes: 8

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