On Sun, 7 Jul 2019 at 22:00, Jonathan Wakely wrote:
>
> On Sun, 7 Jul 2019 at 21:18, Paul Smith <psmith@xxxxxxx> wrote:
> >
> > On Sun, 2019-07-07 at 20:31 +0100, Jonathan Wakely wrote:
> > > > It might be useful to at least discuss this in the
> > > > docs, although I suppose systems using glibc <2.17 are getting more
> > > > rare every day.
> > >
> > > There are performance penalties to using it too, so it's not just a
> > > case of saying "hey, you should use this!"
> > >
> > > If you link to librt on GNU/Linux then you get a dependency on
> > > libpthread which causes libstdc++ to always assume your program is
> > > multithreaded, and use atomic ops for reference counting even in
> > > single-threaded programs.
> >
> > Yes, that information is presented in the docs, which is good. But I
> > think the other side of this (that selecting "rt" on older glibc
> > implementations will give a 22-24% performance increase when calling
> > steady_clock() / system_clock()) should also be mentioned.
How's this?
--- a/libstdc++-v3/doc/xml/manual/configure.xml
+++ b/libstdc++-v3/doc/xml/manual/configure.xml
@@ -166,18 +166,24 @@
<varlistentry><term><code>--enable-libstdcxx-time=OPTION</code></term>
<listitem><para>Enables link-type checks for the availability of the
- clock_gettime clocks, used in the implementation of [time.clock],
- and of the nanosleep and sched_yield functions, used in the
+ <function>clock_gettime</function> clocks, used in the implementation
+ of [time.clock], and of the <function>nanosleep</function> and
+ <function>sched_yield</function> functions, used in the
implementation of [thread.thread.this] of the 2011 ISO C++ standard.
The choice OPTION=yes checks for the availability of the facilities
in libc and libposix4. In case it's needed the latter is also linked
- to libstdc++ as part of the build process. OPTION=rt also searches
- (and, if needed, links) librt. Note that the latter is not always
- desirable because, in glibc, for example, in turn it triggers the
- linking of libpthread too, which activates locking, a large overhead
- for single-thread programs. OPTION=no skips the tests completely.
+ to libstdc++ as part of the build process. OPTION=rt also checks in
+ librt (and, if it's needed, links to it). Note that linking to librt
+ is not always desirable because for glibc it requires linking to
+ libpthread too, which causes all reference counting to use atomic
+ operations, resulting in a potentially large overhead for
+ single-threaded programs. OPTION=no skips the tests completely.
The default is OPTION=auto, which skips the checks and enables the
features only for targets known to support them.
+ For Linux targets, if <function>clock_gettime</function> is not used
+ then the [time.clock] implementation will use a system call to access
+ the realtime and monotonic clocks, which is significantly slower than
+ the C library's <function>clock_gettime</function> function.
</para>
</listitem></varlistentry>
> > Unless you're building for a very specific target/need, you likely will
> > want your compiler to be able to be able to create multi-threaded
> > programs.
>
> I'm not sure what you mean here. The compiler is able to create
> multi-threaded programs either way. The issue is whether
> single-threaded programs pay a cost that's only needed by
> multi-threaded programs or not. With the "rt" option you get a
> compiler that is not able to use a libstdc++ optimisation normally
> enabled for single-threaded programs.
