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From: Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>
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Subject: [libc/string] State of PAGE_COPY_FWD / PAGE_COPY_THRESHOLD
Message-Id: <E122D105-776D-4B9B-A15C-0A7C24BC0910@linaro.org>
Date: Tue, 01 Nov 2016 09:28:00 -0000
To: GNU C Library <libc-alpha@sourceware.org>
X-SW-Source: 2016-11/txt/msg00004.txt.bz2

I wanted to check performance impact of using linux zero page sharing in ca=
lls to memset (PTR, 0, SIZE).  I remembered seeing PAGE_COPY_FWD_MAYBE and =
PAGE_COPY_THRESHOLD in string/memcpy.c, and my plan was to copy this logic =
to an experimental memset() implementation.

Closer inspection of the current code showed that only Mach port attempted =
to use full-page copying in memcpy.c, but now even the Mach port disables i=
t.  The net result is that code in string/memcpy.c, as well as parts of hea=
ders sysdeps/generic/pagecopy.h and sysdeps/generic/memcopy.h are dead code.

=46rom the above we have 2 questions:
1. Is it possible to use full-page copy (with COW) in the Linux glibc port =
for memcpy() and/or memset(0)?

2. If not, then is there any reason to keep the dead code around or should =
we clean it up?

--
Maxim Kuvyrkov
www.linaro.org