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Subject: Re: [RFC PATCH] getcpu_cache system call: caching current CPU
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Date: Sat, 18 Jul 2015 00:43:53 +0200
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On Fri, Jul 17, 2015 at 04:03:30PM +0000, Mathieu Desnoyers wrote:
> ----- On Jul 17, 2015, at 6:58 AM, Ond=C5=99ej B=C3=ADlka neleai@sezn=
am.cz wrote:
> [...]
> >> If we invoke this per-thread registration directly in the glibc NP=
TL
> >> implementation,
> >> in start_thread, do you think it would fit your requirements ?
> >>
> > A generic solution would be adding eager initialization of thread_l=
ocal
> > variables which would fix more performance problems.
>=20
> Concretely, what code would be in charge of this initialization ?
>=20
> What changes does "eager initialization" imply from the current
> implementation ?
>
Now if you write

class foo{
public:
  foo(){
    printf("init\n");
  }
  foo(const foo &x){
    printf("init\n");
  }
  int bar()
  {
    return 32;
  }
};
thread_local class foo x;

Then constructor isn't called in all threads, only main as its lazily
initialized. You need to call x.bar() in each thread to cause
initialization.

> > Second would be write patch to libc adding function
> > pthread_create_add_hook_np to register function that would be ran a=
fter
> > each thread creation.
>=20
> Do you suggest that this callback should be registered once per threa=
d,
> or somehow attached to pthread_attr_t ? As maintainer of lttng-ust,
> where we need some control over our own threads, but where we want mi=
nimal
> impact on the overall application, I would really prefer if we can at=
tach
> the callback to pthread_attr_t.
>=20
Could you elaborate. I was suggesting per-thread callback, what do you
want to do with pthread_attr_t?