Subject: Re: bug#56342: TRAMP (sh) issues way too many commands, thus being very slow over high-ping networks

From: Michael Albinus <[email-masked]>
Date: Sun, 03 Jul 2022 20:47:10 +0200

Paul Pogonyshev <[email-masked]> writes:

Hi Paul,

> Maybe you could then single out commands that are supposed to return
> result nearly-instantly anyway, e.g. `stat' or `test' (though the
> latter I have suggested to replace with `stat' before). If they fail
> to produce the result in 10 seconds, the connection can be assumed
> dead, just as with `echo are you awake'. Commands that involve
> transmitting (potentially) large amount of data this or that way can
> work as now. So, you would usually avoid one extra command (which with
> a high ping means sth. like 0.2 s, already pretty noticeable in UI)
> and achieve the same result.
>
> Sanity checks are good. but not if they visibly slow down normal
> operation.

I will see whether I can do something along these lines.

> It might be easier to convince the rest of Emacs developers to use an
> extra variable to possibly skip `file-exists-p' on only remote files
> than to remove the call altogether. Though in either case it is
> required that `file-missing' error is handled properly which is,
> incidentally, easier to test if the call to `file-exists-p' is
> dropped. A target here would be `insert-file-contents' and whatever it
> calls, and this is a part of Emacs, not some external library.

Tramp has an own implementation of insert-file-contents, called
tramp-handle-insert-file-contents. And indeed, file-exists-p is called
in order to raise the file-missing error if needed. I have no idea how
we could generate this otherwise. Perhaps changing the order: First try
to insert the file contents, and if this errs out, a call with
file-exists-p in order to raise the error. I'll play with this.

OTOH, with longer cache expiration time (see below), file-exists-p
doesn't cost a roundtrip.

> Yet another idea: removing temporary should be done asynchronously (I
> haven't checked, maybe it's already the case, but likely not). The
> caller doesn't really care about call result and even if it has
> succeeded. Again, not sure if this is easy to achieve interface-wise,
> maybe it's from a higher level. But I suspect it's something like
> `with-temp-file' and can be optimized: one-time optimization are worth
> it even if that costs readability.

Doing it asynchronously would require a second connection to the remote
host. Performance would rather degrade.

One thing you could do w/o code change is to change the value of
remote-file-name-inhibit-cache. Its default value is 10, meaning caches
expire already after 10 seconds. With your slow connection, a higher
value should help already.

> Paul

Best regards, Michael.