Date: Thu, 28 Jun 2012 21:00:47 +0800 From: David Xu <listlog2011@gmail.com> To: davidxu@freebsd.org Cc: Konstantin Belousov <kostikbel@gmail.com>, Attilio Rao <attilio@freebsd.org>, svn-src-all@freebsd.org, src-committers@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r237660 - head/lib/libc/gen Message-ID: <4FEC557F.4080807@gmail.com> In-Reply-To: <4FEC1AAB.6070408@gmail.com> References: <201206272032.q5RKWjvt031174@svn.freebsd.org> <4FEBB8C9.8070006@gmail.com> <CAJ-FndBdpP4hFPyqHaU6GucXeGi3WT4d%2Bpu-9hCNEpAPZAZQgg@mail.gmail.com> <4FEBC0A2.3010708@gmail.com> <CAJ-FndD9Hc_KvHe8vC3usVDZh7MYb51gtXsHgmins8dBC3mu_w@mail.gmail.com> <4FEBC70F.40408@gmail.com> <20120628075301.GS2337@deviant.kiev.zoral.com.ua> <4FEC1AAB.6070408@gmail.com>
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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<listlog2011@gmail.com>: >>>>> On 2012/6/28 10:21, Attilio Rao wrote: >>>>>> 2012/6/28, David Xu<listlog2011@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(). Regards, David Xu
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