From: Brian Inglis <Brian.Inglis@SystematicSw.ab.ca>
To: cygwin@cygwin.com
Subject: Re: thread_local performance using g++ for cygwin
Date: Mon, 06 May 2019 20:10:00 -0000 [thread overview]
Message-ID: <36b16ae1-fc0c-a598-480a-ffedad690a76@SystematicSw.ab.ca> (raw)
In-Reply-To: <alpine.WNT.2.00.1905052026250.14000@panamint>
On 2019-05-06 01:09, Arthur Norman wrote:
> The attached code tried two loops each of which just calls a function that
> increments an integer variable. One loop is a simple variable, the other has the
> thread_local qualifier. I put in ugly annotations to prevent g++ from inlining
> the functions even though I compile with -O3, but in real cases separate
> compilation forces each TL access to be independent.
> The timing as between the two cases is EXTREME on cygwin (both 32 and 64-bit)
> however g++ on Linux and the Microsoft compiler on Windows both manage to keep
> the base of thread-local regions in a segment register in such a way that the
> thread_local overhead is minimal. The cygwin thread_local overhead is large
> enough to be very visible in my code as a whole. I can see that changing to use
> a segment register might be a painful API change even if it was feasible, but
> has there been any consideration of it?
> Note that x86_64-w64-mingw32-g++ and clang also do not use the segment register
> so suffer the significant speed penalty, so maybe it would be hard to match what
> Microsoft manage?
>
> Sample output:
> Â Â Â simple 1.265
> Â Â Â thread_local 33.219
See:
https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;f=winsup/cygwin/how-cygtls-works.txt;a=blob
and you may want to compare the gcc default options and -S assembler output for
your test case on Linux and Cygwin, and perhaps also any glibc and newlib TLS
support functions called: running your Linux tests under some WSL distro will
even out OS kernel differences.
My own tests on Win 10.0.17763.437 1809 are worse than yours:
$ g++ -O3 -o tltime.{bin,cpp}
$ ./tltime.bin
simple 1.60938
thread_local 1.95312
$ uname -srvmo
Linux 4.4.0-17763-Microsoft #379-Microsoft Wed Mar 06 19:16:00 PST 2019 x86_64
GNU/Linux
$ head /etc/os-release
PRETTY_NAME="Debian GNU/Linux 9 (stretch)"
NAME="Debian GNU/Linux"
VERSION_ID="9"
VERSION="9 (stretch)"
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
$ g++ -O3 -o tltime.{exe,cpp}
$ ./tltime.exe
simple 1.608
thread_local 53.25
$ uname -srvmo
CYGWIN_NT-10.0 3.0.7(0.338/5/3) 2019-04-30 18:08 x86_64 Cygwin
$ head /etc/os-release
PRETTY_NAME="Cygwin 64 3.0.7 2019-04-30"
NAME=Cygwin
ID=cygwin
ID_LIKE=msys mingw
VARIANT="64"
VARIANT_ID="x86_64"
VERSION="3.0.7 (0.338/5/3) 2019-04-30 18:08"
VERSION_ID="3.0.7"
BUILD_ID="0.338/5/3 2019-04-30 18:08"
CPE_NAME="cpe:/a:cygwin:cygwin:3.0.7::~~~~x64~Windows%3e%3d6.0"
--
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada
This email may be disturbing to some readers as it contains
too much technical detail. Reader discretion is advised.
--
Problem reports: http://cygwin.com/problems.html
FAQ: http://cygwin.com/faq/
Documentation: http://cygwin.com/docs.html
Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
prev parent reply other threads:[~2019-05-06 20:10 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-06 7:09 Arthur Norman
2019-05-06 20:10 ` Brian Inglis [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=36b16ae1-fc0c-a598-480a-ffedad690a76@SystematicSw.ab.ca \
--to=brian.inglis@systematicsw.ab.ca \
--cc=cygwin@cygwin.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).