Date: Sat, 3 Feb 2024 19:59:25 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Mateusz Guzik <mjguzik@gmail.com> Cc: David Chisnall <theraven@freebsd.org>, Brooks Davis <brooks@freebsd.org>, current@freebsd.org Subject: Re: libc/libsys split coming soon Message-ID: <Zb5-_aTnZwrtG6_j@kib.kiev.ua> In-Reply-To: <CAGudoHEAxoqjmKZ91zwVa2BFjrkZriqZW_pLtNnQ5207yVr2Cg@mail.gmail.com> References: <Zb1tTz5LXuVQ5Caj@spindle.one-eyed-alien.net> <CAGudoHGoSW4Zwa9PT_O7AeQX%2B8Q9PKmoQBm9HrmdKD5PNfE0uw@mail.gmail.com> <7E8133B7-4BD5-42AB-8B16-A10F59295F28@FreeBSD.org> <CAGudoHEAxoqjmKZ91zwVa2BFjrkZriqZW_pLtNnQ5207yVr2Cg@mail.gmail.com>
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On Sat, Feb 03, 2024 at 12:12:35PM +0100, Mateusz Guzik wrote: > On 2/3/24, David Chisnall <theraven@freebsd.org> wrote: > > On 3 Feb 2024, at 09:15, Mateusz Guzik <mjguzik@gmail.com> wrote: > >> > >> Binary startup is very slow, for example execve of a hello world > >> binary in a Linux-based chroot on FreeBSD is faster by a factor of 2 > >> compared to a native one. As such perf-wise this looks like a step in > >> the wrong direction. It is the right change to improve modularity and the structure of the code. > > > > Have you profiled this? Is the Linux version using BIND_NOW (which comes > > with a load of problems, but it often the default for Linux systems and > > reduces the number of slow-path entries into rtld)? Do they trigger the > > same number of CoW faults? Is there a path in rtld that’s slower than the > > equivalent ld-linux.so path? Linux version probably benefits from pre-linking, which might have the side-effect of breaking semantic into as if BIND_NOW is activated. > > > > I only profiled FreeBSD, it was 4 years ago. I have neither time nor > interest in working on this. > > Relevant excerpts from profiling an fexecve loop: > > Sampling what syscalls was being executed when in kernel mode > (or trap): > > syscalls: > pread 108 > fstat 162 > issetugid 250 > sigprocmask 303 > read 310 > mprotect 341 > open 380 > close 1547 > mmap 2787 > trap 5421 > [snip] > In userspace most of the time is spent here: > ld-elf.so.1`memset 406 > ld-elf.so.1`matched_symbol 431 > ld-elf.so.1`strcmp 1078 > ld-elf.so.1`reloc_non_plt 1102 > ld-elf.so.1`symlook_obj 1102 > ld-elf.so.1`find_symdef 1439 > > find_symdef iterates a linked list, which I presume induces strcmp calls > due to unwanted entries. > [snip] So strcmp() is almost 1:1 with reloc_non_plt and/or symlook_obj. It demonstrates that the ELF hash (perhaps GNU hash, but I do not remember how long do we have it) provides very good distribution.home | help
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