Removing The Global GC Lock: Largest Plausible Number of Threads?

[Daniel Gibson]

Am 12.05.2011 06:33, schrieb Jonathan M Davis:
> On 2011-05-11 21:14, dsimcha wrote:
>> I'm thinking about ways to remove the global lock from the garbage
>> collector for most small allocations.  I'm basically thinking of making
>> the free lists thread local.  Every scheme I can come up with that
>> doesn't require a radical overhaul of the current implementation
>> requires every thread having a unique ID.  I want to do this as simply
>> and efficiently as possible, preferably using dense integers.  Is it
>> reasonable to assume that no program will ever need more than 2 ^^ 16
>> thread (about 65,000) simultaneously so that I can store these indices
>> as ushorts?  If a program creates a lot of short-lived threads, the
>> indices will be recycled, so having a huge number of threads
>> non-simultaneously is not a problem.
> 
> I don't think that you can legally create that many threads on a typical OS. 
> I'd have to check, but as I recall, the typical limit is much lower than that 
> - still in the tens of thousands, I think, but not that high.
> 
> - Jonathan M Davis

On Windows you get a DWORD (32bit int AFAIK) from GetCurrentThreadId(),
so that seems to be a technical limit there.
On Linux there's pthread_t which seems to be a 32bit uint on x86 and
64bit ulong on amd64.

However, see:
http://stackoverflow.com/questions/344203/maximum-number-of-threads-per-process-in-linux
It seems like it's limited by the available stack space (especially on
32bit systems)
Similar on Windows:
http://blogs.msdn.com/b/oldnewthing/archive/2005/07/29/444912.aspx

Cheers,
- Daniel