Re: RTS's (old?) invariant regarding OS blocking

On Thu, Mar 03, 2016 at 09:52:07AM +0000, Sergei Trofimovich wrote:

On Wed, 2 Mar 2016 16:38:56 +0200 Dan Aloni wrote:

...

Hi,

While trying to gain insights into the RTS, I've noticed the following in the Wiki page [1] on the topic of the scheduler:

Invariant: a task that holds a capability is not blocked in the operating system.

This makes some parts of the system simpler - for example, we can use spin locks that spin indefinitely, because we can ensure that the spin lock is only held by a currently executing CPU, and will therefore be released in a finite (and short) amount of time.

Does it still apply to modern day GHC, or was it addressed by [2]?

It still does apply. Foreign calls are by default 'safe' and executed after Capability is released to a separate OS thread. Capability release is needed as foreign calls can reenter haskell RTS.

Thanks for the elborate example - however my concerns extend beyond the safe vs. unsafe FFI. I was concerned about the spin lock, and about hard host kernel scheduling assumptions thereof. I looked into the GHC code now, and the locks in Capability.c on Linux seem to translate to pthread_mutex_lock(), which is a futex. This is a relief. Does the wiki need updating? -- Dan Aloni