https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96484
Bug ID: 96484
Summary: Horrible performance of std::read_symlink
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: terra at gnome dot org
Target Milestone: ---
Created attachment 49004
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49004&action=edit
Preprocessed ttt.C
This program takes ~0.25s per round in the loop for a total of minutes. It
should take millisecons in total.
To observe the problem, "foo" needs to be a large file -- more than 500M in my
case -- that is not a symlink. Mine is on an NFS server.
#include <filesystem>
#include <iostream>
namespace fs = std::filesystem;
int
main()
{
for (int i = 0; i < 1000; i++) {
std::error_code ec;
std::cerr << i << ": " << fs::read_symlink ("foo", ec) << std::endl;
}
}
The problem is in the implementation of read_symlink. First an lstat call is
done (fine), but then the st_size field is used as a hint for the symlink's
size. There is no check that the file is a symlink at this point, so in my
case a 500M buffer is allocated via an mmap system call and the kernel takes
its sweet time.
Note: a file with size 4G-1 on a 32-bit system might produce an infinite loop
as the buffer will have size 0 and get doubled from that.
Suggestion 1: if the lstat doesn't show a symlink, set the error code
right then and exit.
Suggestion 2: cap st_size at something reasonable and let the loop handle it if
it needs to be bigger.
# /usr/local/products/gcc/10.1.0/bin/g++ -v
Using built-in specs.
COLLECT_GCC=/usr/local/products/gcc/10.1.0/bin/g++
COLLECT_LTO_WRAPPER=/usr/local/products/gcc/10.1.0/lib/gcc/x86_64-suse-linux/10.1.0/lto-wrapper
Target: x86_64-suse-linux
Configured with: ../../gcc-10.1.0/configure --enable-languages=c,c++,fortran
--enable-targets=x86_64-suse-linux,i686-suse-linux
--prefix=/usr/local/products/gcc/10.1.0 --with-gnu-as
--with-as=/usr/local/products/gcc/binutils-2.32/bin/as --with-gnu-ld
--with-ld=/usr/local/products/gcc/binutils-2.32/bin/ld --enable-threads=posix
--enable-shared --enable-__cxa_atexit --enable-libstdcxx-allocator=pool
x86_64-suse-linux
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.1.0 (GCC)
# uname -a
Linux monsterd09 5.0.13-1-default #1 SMP Sun May 5 15:48:04 UTC 2019 (b11e2d7)
x86_64 x86_64 x86_64 GNU/Linux
