Re: `start-process' awfully slow

[York Zhao <gtdplatform@gmail.com>]

>> As mentioned above, when `call-process' is used, it takes 2 seconds
>> for Clang to generate 15,000 lines of completion results and then the
>> results gets received by Emacs in a temporary buffer. 2 seconds is
>> painful but still OK.
>
> Which part of those 2 seconds is due to clang, and which part to Emacs?

I don't know exactly the proportion of time being consumed, but I think most of
the 2 seconds are consumed by the clang program. Is there an easy way to know
exactly how long haven been consumed by Emacs to receive the output from clang?

>> Because of the slowness, I tried to add the functionality to be able
>> to call Clang asynchronously so that the keyboard input will not be
>> blocked while awaiting the completion results. What had driven me
>> crazy is that it takes more than 15 seconds for Emacs to receive the
>> 15,000 lines of completion results if `start-process' is is being used
>> to call the Clang executable. During this 15 seconds I was not typing
>> anything so Emacs is "idling". The question is why `start-process'
>> takes 15 seconds while `call-process' takes only 2 seconds, and what
>> can I do about it?

> I assume those lines aren't terribly long.

The longest line had exceeded 200, I noticed this because there had been a time
I had to set `line-number-display-limit-width' to over 200 (default being 200)
in order for `column-number-mode' to be able to display the column number if the
maximum length of lines in a buffer exceeds 200.

> so we might be talking about less than 1MB of data

The size is actually 1.4MB.

> I suggest you report it as a bug (via M-x report-emacs-bug), trying to provide
> as much info as possible to make it reproducible (e.g. replacing clang with a
> "cat" that simply outputs those 15K lines).

Replacing "clang" with "cat" is a good idea to reproduce the problem. I might
have to report the bug, thanks for your suggestion.

Thanks,

York

On Sun, Apr 14, 2013 at 2:24 PM, Stefan Monnier
<monnier@iro.umontreal.ca> wrote:
>> As mentioned above, when `call-process' is used, it takes 2 seconds
>> for Clang to generate 15,000 lines of completion results and then the
>> results gets received by Emacs in a temporary buffer. 2 seconds is
>> painful but still OK.
>
> Which part of those 2 seconds is due to clang, and which part to Emacs?
>
>> Because of the slowness, I tried to add the functionality to be able
>> to call Clang asynchronously so that the keyboard input will not be
>> blocked while awaiting the completion results. What had driven me
>> crazy is that it takes more than 15 seconds for Emacs to receive the
>> 15,000 lines of completion results if `start-process' is is being used
>> to call the Clang executable. During this 15 seconds I was not typing
>> anything so Emacs is "idling". The question is why `start-process'
>> takes 15 seconds while `call-process' takes only 2 seconds, and what
>> can I do about it?
>
> There's no doubt that start-process has to work harder than
> call-process, but 13s to transfer 15K lines (I assume those lines aren't
> terribly long, so we might be talking about less than 1MB of data) is
> too long.  I suggest you report it as a bug (via M-x report-emacs-bug),
> trying to provide as much info as possible to make it reproducible
> (e.g. replacing clang with a "cat" that simply outputs those 15K lines).
>
>
>         Stefan