[M3devel] thread local storage, but in C pieces of runtime... (jay, tony?)

[Dragiša Durić]

AFAIK, Linux ABI uses dedicated register (one of segment registers) to access TLS. It's setting is part of context switch, and one is only supposed to use it read-only. No kernel space is involved in accessing, and was not since NPTL was introduced.

Old and FreeBSD does ring bells :). Linux pioneered lightweight processes/threads in free *nix world, and others followed. If your test was near enough in time to 2004-5-6, then probably it was a time when FreeBSD played catch-up.

Also, on Linux since 2004, uncontested lock is user-space only. It probably involves a barrier of some kind, but it is under 0.5microsecond in worst case, as opposed to 20-50microsecond for kernel space operation.

So, it is efficient :).

On May 10, 2013, at 5:22 PM, mika at async.caltech.edu wrote:

> This is just one data point, but I remember testing the performance of thread locals
> on pthreads, probably on an old FreeBSD system, and the performance was horrendously
> bad.  There was a system call involved in obtaining the pointer to the thread-local
> area.  I was had some clever algorithms I wanted to use thread locals for (since you
> can do all sorts of things without locks then), but I gave up on it since it was way
> more expensive than a lock on that system...
> 
>     Mika

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://m3lists.elegosoft.com/pipermail/m3devel/attachments/20130510/c6a83820/attachment-0002.html>