Re: lstreams (was Re: package badness)
Kyle Jones <kyle_jones(a)wonderworks.com> writes:
Before lstreams, stdio was not used for saving buffers. Prior to
19.12, XEmacs saved a buffer by doing two write() operations--- the
first from beginning of buffer to the gap and the second from the
gap to end of buffer. Fast. At present, we copy data from an Emacs
buffer to stack buffer. Then from a stack buffer to an lstream
buffer. Then finally we write() it to disk. Slow. The same sort
of gup is done in insert-file-contents-internal.
Point taken. :-( insert-file-contents-internal should probably use
mmap() on systems that support it.
Why would lstreams be slower than stdio for reading Lisp? Lstreams
do more. Stdio only allows you to unget one character. lstreams
allow unlimited unget, and so an extra index must be checked for
every call to (the macroized) Lstream_getc. Lstream_getc is run for
every char read from a Lisp file. The extra instructions add up.
Lstream_getc also increments a byte counter that stdio almost
certainly would not be maintaining. More instructions. This is
from just eyeballing the code, so I could be wrong.
Someone should benchmark lstream against stdio to be sure. You may
well be right that these things add up. But then again, you might not
be -- it could be the case that 90% of the time is spent in IO
whatever else we do.