libkse and SMP (was Re: USB bulk read & pthreads)

[Terry Lambert]

Daniel Eischen wrote:
> This is just because rtld-elf is not thread safe (or written
> so that locks are not needed as you claim).

No.  There is plenty of code that falls into this category; as
soon as you finish fixing rtld, you will reveal other problems.
Mark my words; you cannot make user space thread safe for all
eventualities, with the user process never having to know
anything about mutexes that the pthreads model expects the user
program to hold in order to ensure against race conditions.


> > The second of these is that the libkse model is M:N, with N being
> > defaulted to 1.  If you want more kernel threads, you have to ask
> 
> N is defaulted to the number of CPUs that you have.  Libkse
> will create as many KSEs as there are CPUs (or whatever
> kern.threads.virtual_cpu is set to); these will be KSEs
> that run scope process threads.  Scope system threads
> get their own KSE/KSEG pair without regard to number of
> CPUs or kern.threads.virtual_cpu.
> 
> Trying to exceed the number of CPUs with kern.threads.virtual_cpu
> will not work unless you also set kern.threads.debug=1.  You
> really shouldn't need to do this, but we use it for testing
> and debugging.

This is handy to know; so basically, my expectation from
reading the code around PTHREAD_SCOPE_SYSTEM was correct:
a single CPU system with PTHREAD_SCOPE_PROCESS (the default)
can still get itself blocked in the kernel by a single
blocking call (as in the USB bulk read device issue).

So he can avoid using the sysctl's, which would be evil, but
he could test without modifying his program, if he had to.

Thanks for the info!
-- Terry