[Python-Dev] first post introduction and question regarding lto

[Gregory P. Smith]

I don't know whether it is beneficial or not - but having the capability to
build LTO without PGO seems reasonable. I can review any pull requests
altering configure.ac and Makefile.pre.in to make such a change.

- at gpshead

On Mon, Aug 7, 2017 at 5:08 PM Soldea, Octavian <octavian.soldea at intel.com>
wrote:

> Hello
>
> Thank you for your messages. I am not quite sure I understood, therefore,
> I would like to ask if you see beneficial to have the option of lto
> separately?
>
> Best regards,
>   Octavian
>
>
>
>
>
>
> -----Original Message-----
> From: Wang, Peter Xihong
> Sent: Monday, August 7, 2017 5:00 PM
> To: Victor Stinner <victor.stinner at gmail.com>; Soldea, Octavian <
> octavian.soldea at intel.com>
> Cc: Python-Dev at python.org
> Subject: RE: [Python-Dev] first post introduction and question regarding
> lto
>
> We evaluated different tests before setting down and proposing regrtest
> suite for PGO training, including using OpenStack benchmarks for OpenStack
> applications.  The regrtest suite was found consistently giving the best in
> terms of performance across applications/workloads.
>
> So I would hesitate to remove any test from the training suite (for the
> purpose of reducing build time), unless we are absolute sure it will not
> hurt performance across the board.
>
> Thanks,
>
> Peter
>
> -----Original Message-----
> From: Python-Dev [mailto:python-dev-bounces+peter.xihong.wang=
> intel.com at python.org] On Behalf Of Victor Stinner
> Sent: Monday, August 07, 2017 4:43 PM
> To: Soldea, Octavian <octavian.soldea at intel.com>
> Cc: Python-Dev at python.org
> Subject: Re: [Python-Dev] first post introduction and question regarding
> lto
>
> I don't think that PGO compilation itself is slow. Basically, I expect
> that it only doubles the compilation time, but compiling Python takes less
> than 1 minute usually. The slow part is the profiling task: run the full
> Python test suite, which takes at least 20 minutes. The tests must be run
> sequentially.
>
> It would help to reduce the number of tests run in the profiling task.
> We should also maybe skip tests which depend too much on I/O to get more
> reproductible PGO performances.
>
> Victor
>
> 2017-08-08 1:01 GMT+02:00 Soldea, Octavian <octavian.soldea at intel.com>:
> > Hello
> >
> >
> >
> > Gregory: Thank you for your message. I really appreciate it.
> >
> >
> >
> > In my opinion the use of lto only compilation mode can be of benefit
> > from two reasons at least:
> >
> > 1.       Lto only can provide a platform for experimentations. Of
> course, it
> > seems to be very application specific.
> >
> > 2.       Lto compilation is faster than an entire pgo + lto mode. While
> pgo
> > can take a lot of time, lto can provide significant optimizations, as
> > compared to baseline, at the cost of less compilation time. Of course,
> > lto is time consuming. To the best of my knowledge, compiling with lto
> > only is shorter than combining pgo and lto.
> >
> >
> >
> > In this context, I will be more than happy to receive more feedback
> > and opinions.
> >
> >
> >
> > Best regards,
> >
> >   Octavian
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > From: Gregory P. Smith [mailto:greg at krypto.org]
> > Sent: Monday, August 7, 2017 3:12 PM
> > To: Soldea, Octavian <octavian.soldea at intel.com>;
> > Python-Dev at python.org
> > Subject: Re: [Python-Dev] first post introduction and question
> > regarding lto
> >
> >
> >
> > I've personally never seen a situation where PGO is not desired yet
> > some other fancier optimization such as LTO is. When do you encounter
> > people wanting that?  PGO always produces a 10-20% faster CPython
> interpreter.
> >
> >
> >
> > I have no problem with patches enabling an LTO only build for anyone
> > who wants one if they do not already exist.  At the moment I'm not
> > sure which LTO toolchains actually work properly.  We removed it from
> > inclusion in --enable-optimizations because it was clearly broken in
> > some common environments.
> >
> >
> >
> > If LTO without PGO is useful, making it work is probably just a matter
> > of making sure @LTOFLAGS@ show up on the non-PGO related
> > Makefile.pre.in targets as well as updating the help text for
> --with-lto in configure.ac.
> > [untested]
> >
> >
> >
> > -gps
> >
> >
> >
> >
> > Python-Dev mailing list
> > Python-Dev at python.org
> > https://mail.python.org/mailman/listinfo/python-dev
> > Unsubscribe:
> > https://mail.python.org/mailman/options/python-dev/greg%40krypto.org
> >
> >
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> >
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