449222 - MMgc spends too many TLS slot entries

Status: VERIFIED WONTFIX

(Tamarin Graveyard :: Garbage Collection (mmGC), defect, P3)

Description

[Michael Daumling]

XP has 1088 Thread Local Storage (TLS) slots per process, and Mac OSX seems to have only 128 for pthread_key_create (search for PTHREAD_KEYS_MAX). TT has a class that reserves a slot when instantiated (in GCThreadLocal.h), which wastes TLS entries; this is a bad policy for a shared component.

GCThreadLocal.h needs to be rewritten to use a single TLS slot that contains a pointer to a structure containing all of the thread local data. The routine can also be rewritten that, in single-threaded mode, the getter for this structure just returns a pointer to a static instance, which speeds up accesses considerably.

Comment 1

[Michael Daumling]

Correction: Mac OSX has PTHREAD_KEYS_MAX increased from 128 to 512.

Comment 2

[Michael Daumling]

Sent too early: The maximum of 512 is valid for OSX 10.5++.

Comment 3

[Dan Schaffer]

triaging.  send to qrb.

Comment 4

[Trevor Baker]

tom, can you take a look?

Comment 5

[Tommy Reilly]

Its a valid bug but we don't use this class much so the need to fix is low.

Comment 6

[Trevor Baker]

Is this only affecting TT (as written) or does it affect redux/central as well?

Comment 7

[Tommy Reilly]

No this affects all versions of tamarin

Comment 8

[Lars T Hansen]

Tamarin currently uses four thread local slots, and that's being reduced to three when the sampler has been fixed.  Two of the remaining three uses are somewhat gratuitous; they could be hung off the EnterFrame.  Generally, thread local slots don't work well because VMs can be moved from thread to thread on a system that has a pool of VMs that it schedules on a smaller number of threads.