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bug#59321: ice-9's open-input-pipe is unexpectedly slow on some systems
From: |
Andrew Whatson |
Subject: |
bug#59321: ice-9's open-input-pipe is unexpectedly slow on some systems |
Date: |
Fri, 18 Nov 2022 14:49:05 +1000 |
Hi Nate,
Thanks for reporting this! I'm able to reproduce on my systems, and
have identified the culprit:
> strace -c -f guile -c '(begin (use-modules (ice-9 popen) (ice-9
rdelim)) (display (read-delimited "" (open-input-pipe "ls"))))'
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ------------------
93.38 0.309261 1 250107 249994 close
4.61 0.015268 135 113 21 futex
0.52 0.001730 4 430 270 newfstatat
0.38 0.001263 6 183 mmap
0.29 0.000953 4 197 92 openat
0.16 0.000542 180 3 execve
<...snip...>
These close calls are from the start_child routine in libguile/posix.c:
static pid_t
start_child (const char *exec_file, char **exec_argv,
int reading, int c2p[2], int writing, int p2c[2],
int in, int out, int err)
{
int pid;
int max_fd = 1024;
#if defined (HAVE_GETRLIMIT) && defined (RLIMIT_NOFILE)
{
struct rlimit lim = { 0, 0 };
if (getrlimit (RLIMIT_NOFILE, &lim) == 0)
max_fd = lim.rlim_cur;
}
#endif
<...snip...>
/* Close all file descriptors in ports inherited from the parent
except for in, out, and err. Heavy-handed, but robust. */
while (max_fd--)
if (max_fd != in && max_fd != out && max_fd != err)
close (max_fd);
<...snip...>
On my system, `ulimit -n` shows the maximum number of open files as
250000, explaining the 250000 calls to close shown by strace.
Testing a build of guile with the max_fd loop commented out shows the
expected performance:
# unmodified guile
> time guile -c '(begin (use-modules (ice-9 popen) (ice-9 rdelim))
(display (read-delimited "" (open-input-pipe "ls"))))'
real 0m0.202s
# guile patched to remove max_fd loop
> time guile -c '(begin (use-modules (ice-9 popen) (ice-9 rdelim))
(display (read-delimited "" (open-input-pipe "ls"))))'
real 0m0.056s
Forcibly closing file descriptors like this shouldn't be necessary if
the application has properly opened descriptors with the FD_CLOEXEC
flag. It would be good to get input from some more experienced Guile
hackers on the potential consequences of this change.
Cheers,
Andrew