On 2012/06/28 21:52, Kostik Belousov wrote:
On Thu, Jun 28, 2012 at 4:00 PM, David Xu <listlog2...@gmail.com> wrote:
On 2012/6/28 16:49, David Xu wrote:
On 2012/6/28 15:53, Konstantin Belousov wrote:
On Thu, Jun 28, 2012 at 10:53:03AM +0800, David Xu wrote:
On 2012/6/28 10:32, Attilio Rao wrote:
2012/6/28, David Xu<listlog2...@gmail.com>:
On 2012/6/28 10:21, Attilio Rao wrote:
2012/6/28, David Xu<listlog2...@gmail.com>:
On 2012/6/28 4:32, Konstantin Belousov wrote:
Author: kib
Date: Wed Jun 27 20:32:45 2012
New Revision: 237660
URL: http://svn.freebsd.org/changeset/base/237660
Log:
Optimize the handling of SC_NPROCESSORS_CONF, by using auxv
AT_NCPU
value if present.
MFC after: 1 week
Modified:
head/lib/libc/gen/sysconf.c
Modified: head/lib/libc/gen/sysconf.c
==============================================================================
--- head/lib/libc/gen/sysconf.c Wed Jun 27 20:24:25 2012
(r237659)
+++ head/lib/libc/gen/sysconf.c Wed Jun 27 20:32:45 2012
(r237660)
@@ -42,6 +42,7 @@ __FBSDID("$FreeBSD$");
#include<sys/resource.h>
#include<sys/socket.h>
+#include<elf.h>
#include<errno.h>
#include<limits.h>
#include<paths.h>
@@ -51,6 +52,7 @@ __FBSDID("$FreeBSD$");
#include "../stdlib/atexit.h"
#include "tzfile.h" /* from
../../../contrib/tzcode/stdtime */
+#include "libc_private.h"
#define _PATH_ZONEINFO TZDIR /* from tzfile.h */
@@ -585,6 +587,8 @@ yesno:
case _SC_NPROCESSORS_CONF:
case _SC_NPROCESSORS_ONLN:
+ if (_elf_aux_info(AT_NCPUS,&value, sizeof(value)) == 0)
+ return ((long)value);
mib[0] = CTL_HW;
mib[1] = HW_NCPU;
break;
Will this make controlling the number of CPU online or CPU hotplug
be impossible on FreeBSD ?
If I think about hotplug CPUs I can think of other 1000
problems/races/bad situations to be fixed before this one, really.
These are problems only in kernel, but kib's change is about ABI
between userland and kernel, I hope we don't introduce an ABI which
is not extendable road stone.
I'm not entirely sure I see the ABI breakage here.
It is not breakage, it is the ABI thinks number of online cpu is fixed,
obviously, it is not the case in future unless FreeBSD won't support
dynamic number of online cpus.
If the AT_NCPUS
becames unconvenient and not correct at some point we can just fix
sysconf() to not look into the aux vector anymoe.
If you already know this will be a problem, why do you introduce it
and later need to fix it ?
Please note that
AT_NCPUS is already exported nowadays. I think this is instead a
clever optimization to avoid the sysctl() (usual way to retrieve the
number of CPUs).
But why don't you cache it in libc ? following code is enough:
static int online_cpu;
if (online_cpu == 0)
online_cpu = sysctl
return online_cpu;
Thread did evolved somewhat while I was AFK.
First, please note that the ABI which I designed there is fixable:
if kernel does not export AT_NCPUS at all, then auxv correctly handles
the situation returning an error, and libc falls back to sysctl(2).
Do we really want to bypass sysctl and instead passing all info via auxv
vector ?
I found the sysconf() is a bunch of switch-case, which is already slow,
before
_SC_NPROCESSES_ONLN, it has already a quite number of case branches,
and in your code, it calls _elf_aux_info() which also has some
switch-cases branch,
if you cache smp_cpus in libc, the call for _elf_aux_info is not needed,
and you
don't need code in kernel to passing it either, in any case, the code to
call
sysctl is still needed, so why don't we just use sysctl instead and cache
the result in libc ? this at least can generate small code and a bit
faster after
first call to sysconf(_SC_NPROCESSES_ONLN).
And as a side note, I think we should not put non-critical code into
fork/exec
path, these two functions are rather critical path for any UNIX like
system,
anything slowing down these two functions will affect overall performance,
so we should not waste cpu cycle trying to push data to user mode via auxv
or other ways and yet the data is not used by user code in most time,
such as the number of online cpu. And in rtld-elf or libc, we should not
waste
too much time before executing main().
My motivation for extending auxv vector and to develop auxv.c was
exactly to shave around dozen
of syscalls from the application startup sequence. If you look at the
ktrace output of the binary startup
on RELENG_8 libc, you should note exactly the sysctls to request
ncpus, pagesizes, canary and so on.
In RELENG_9 and HEAD, there are no sysctls in the trace, because the
data is already pre-filled by
kernel for libc consumption.
The sysconf(3) commit you are commenting on was caused by jemalloc(3)
initialization starting using
_sysconf(3) to query ncpu (for older version, which used sysctl
directly, I direct auxv call). So HEAD
has temporary +1 sysctl syscall done on app startup, now it should be
back to zero.
This is a bug of jemalloc, in case __isthreaded is zero, it should not
call sysctl to get number of cpu. I think it has already bypassed
pthread_mutex_lock/unlock if __isthreaded is zero.
In other words, 'we should not put non-critical code into fork/exec
path' exactly contradicts with
proposal to remove auxv, since exec would need to call sysctls to
fetch the same data.
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