Re: Why sourcing a file is not faster than doing a loop with eval, zle -N

[Stephane Chazelas <stephane.chazelas@xxxxxxxxx>]

2017-06-19 08:31:16 -0700, Bart Schaefer:
> On Jun 19,  1:24pm, Stephane Chazelas wrote:
> }
> } There's probably scope for optimisation here, though I can't
> } comment further as I don't know why that signal handling code is
> } there in the first place.
> 
> rt_signprocmask should not be significantly more expensive than an
> assignment to an integer.

Still,

$ time zsh -c 'repeat 100 . ./fsh_cache'  2> /dev/null
zsh -c 'repeat 100 . ./fsh_cache' 2> /dev/null  0.73s user 0.78s system 99% cpu 1.522 total
$ time zsh -c 'repeat 100 eval "$(<fsh_cache)"'  2> /dev/null
zsh -c 'repeat 100 eval "$(<fsh_cache)"' 2> /dev/null  0.80s user 0.04s system 99% cpu 0.848 total

See how the system time falls to almost 0 with the eval variant.
I get the same kind of performance gain if I comment out the
line that eventually calls the rt_signprocmask there.
winch_unblock() (so only for SIGWINCH).

> The signal handling code is there because the shell MUST NOT respond
> instantly to arbitrary signals while doing operations such as token
> interpretation or or memory management -- the signal handlers might
> themselves invoke shell commands/functions and many of those layers
> are not safe for re-entrancy -- but it MUST respond to those signals 
> whenever it may be blocked for an unknown length of time, such as when
> reading from a file descriptor.
> 
> Many years of "I can't interrupt my script when X" or "interrupting
> my script when Y causes a crash" resulted in the current signal
> paradigm.  When the shell was first written, processors weren't fast
> enough and process scheduling not well-threaded enough to expose a
> lot of these issues, but the better our computers get the greater
> the likelyhood of hitting an ever-smaller race condition window, so
> those windows have to be aggressively closed.

I suspected it would be something like that, but here note that
it's done for every byte of the data even though the code is
read in full chunks at a time (by stdio's fgetc)

If you look at the strace output, you see something:

open("/etc/zsh/zshenv", O_RDONLY|O_NOCTTY) = 3
fcntl(3, F_DUPFD, 10)                   = 11
read(11, "# /etc/zsh/zshenv: system-wide ."..., 4096) = 623
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [], 8) = 0
[...]
open("./fsh_cache", O_RDONLY|O_NOCTTY)  = 3
fcntl(3, F_DUPFD, 10)                   = 13
read(13, "zle -N orig-s0.0000060000-r9037-"..., 4096) = 4096
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [CHLD], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [CHLD], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [CHLD], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [CHLD], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [CHLD], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [CHLD], 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [WINCH], [CHLD], 8) = 0
[...]

Most of those rt_sigprocmask are unnecessary.

That defeats a benefit of stdio saving read() systems calls by
reading in chunk if we end up doing one system call per byte
anyway.

-- 
Stephane