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SubjectRe: pthread_create() slow for many threads; also time to revisit 64b context switch optimization?

* Ulrich Drepper <> wrote:

> Hash: SHA1
> Ingo Molnar wrote:
> > i find it pretty unacceptable these days that we limit any aspect of
> > pure 64-bit apps in any way to 4GB (or any other 32-bit-ish limit).
> Sure, but if we can pin-point the sub-archs for which it is the
> problem then a flag to optionally request it is even easier to handle.
> You'd simply ignore the flag for anything but the P4 architecture.

i suspect you are talking about option #2 i described. It is the option
which will take the most time to trickle down to people.

> I personally have no problem removing the whole thing because I have
> no such machine running anymore. But there are people out there who
> have.

hm, i think the set of people running on such boxes _and_ then upgrading
to a new glibc and expecting everything to be just as fast to the
microsecond as before should be miniscule. Those P4 derived 64-bit boxes
were astonishingly painful in 64-bit mode - most of that hw is running
32-bit i suspect, because 64-bit on it was really a joke.

Btw., can you see any problems with option #1: simply removing MAP_32BIT
from 64-bit stack allocations in glibc unconditionally? It's the fastest
to execute and also the most obvious solution. +1 usecs overhead in the
64-bit context-switch path on those old slow boxes wont matter much.

10 _millisecs_ to start a single thread on top-of-the-line hw is quite
unaccepable. (and there's little sane we can do in the kernel about
allocation overhead when we have an imperfectly filled 4GB box for all


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