bug#71355: 30.0.50; [PATCH] Improve performance of buffered output in Eshell

[Jim Porter]

On 6/5/2024 11:58 AM, Eli Zaretskii wrote:
> So now that we agree about this aspect, I ask again: wouldn't it make
> sense to show the text to the user in smaller chunks?  2K characters
> is 2 dozen lines, and I expect users to be somewhat unhappy about
> being presented the text in such large chunks.  That's now what they
> see when invoking 'cat' from the shell prompt outside Emacs.

Ok, I see what you mean. I think the thing users would be unhappy about 
is "long" periods of time between display updates. (If we flush and/or 
redisplay faster than the user's monitor refreshes, those updates are 
wasted.)

For the kinds of output that Eshell's buffered-print is designed for, we 
can get the text we want to print very quickly, so even with a buffer 
size of 2048, we flush more than 60 times a second (testing with "cat" 
and "ls" on a spinny disk). In situations where the buffering caused 
unacceptable delays, a command would either a) not use buffered output 
or b) manually flush at opportune times. I'm not sure those would come 
up in practice though.

Ultimately, in the cases where Eshell does buffered-printing now, the 
thing that limits the user seeing updates is the redisplay throttle, not 
the buffer size.

A smarter version of 'eshell-buffered-print' could flush before the 
buffer was full if enough time has passed, but that would add complexity 
without a lot of immediate benefit. (For example, would we set up a 
timer to flush? I'm not sure how that would interact with the rest of 
this code, which is all synchronous.)