From: Dima Kogan <dima@secretsauce.net>
To: 53041@debbugs.gnu.org
Subject: bug#53041: 29.0.50; TRAMP spins the CPU by polling the child processes without a delay
Date: Wed, 05 Jan 2022 15:03:48 -0800 [thread overview]
Message-ID: <87pmp57pkb.fsf@jpl.nasa.gov> (raw)
Hi. I use TRAMP regularly, and I often see it redline my CPU, which
shouldn't be happening.
The cause in all cases I've seen is TRAMP expecting some output from the
child process, and looking for this output in a delay-less loop. For
instance (tramp-process-one-action) looks like this:
(defun tramp-process-one-action (proc vec actions)
....
(while (not found)
(while (tramp-accept-process-output proc 0))
.... )
The (while (tramp-accept-process-output proc 0)) form does
Read all available data; returns immediately if none is available
So here we spin the CPU until there's some data to look at AND until the
incoming data meets some condition we're looking for. In order to not
spin, at least one of the (tramp-accept-process-output) calls needs to
block. The simplest thing to do to fix this is to replace
(while (tramp-accept-process-output proc 0))
with
(tramp-accept-process-output proc nil)
Here we block until we get SOME data back. I think this is probably
good-enough, since the outer loop will get more data, if it's needed. If
we really want to replace the original logic with blocking, we can do
this instead:
(let (timeout)
(while
(prog1
(tramp-accept-process-output proc timeout)
(setq timeout 0))))
Either one of these makes most of these issues disappear. There are more
places in the code where we call (tramp-accept-process-output ... 0),
and I think they're all wrong: we should always block. I can send a
patch, but let's agree on the approach first. My preference is to
replace all the (while (tramp-accept-process-output proc 0)) with
(tramp-accept-process-output proc nil) unless there's a specific reason
not to.
One easy way to reproduce one such behavior:
1. Start up emacs
2. open /ssh:SERVER:FILE
3. Break the network connection (I'm on a laptop. Leaving the wifi area
is enough)
4. Try to type into the buffer visiting FILE
5. See emacs block the user while spinning the CPU.
Thanks
next reply other threads:[~2022-01-05 23:03 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-05 23:03 Dima Kogan [this message]
2022-01-09 13:46 ` bug#53041: 29.0.50; TRAMP spins the CPU by polling the child processes without a delay Michael Albinus
2022-01-14 8:13 ` Lars Ingebrigtsen
2022-01-14 18:33 ` Dima Kogan
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: https://www.gnu.org/software/emacs/
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87pmp57pkb.fsf@jpl.nasa.gov \
--to=dima@secretsauce.net \
--cc=53041@debbugs.gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://git.savannah.gnu.org/cgit/emacs.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).