Re: [PATCH] gdb: unbuffer all input streams when not using readline

[Andrew Burgess <aburgess@redhat.com>]

Andrew Burgess <aburgess@redhat.com> writes:

> * Simon Marchi via Gdb-patches <gdb-patches@sourceware.org> [2022-01-18 11:26:42 -0500]:
>
>>
>> The change looks ok to me (better than the status quo), given that
>> correctness is more important than performance.
>>
>> I'm just wondering if there's a noticeable performance difference
>> between having the input buffered vs unbuffered.  Calling fgetc with
>> unbuffered input means we do one syscall per character.  With frontends
>> sending tons of commands, it could possibly affect the responsiveness
>> and degrade user experience.  But it's just a guess, we should be able
>> to measure it.
>
> I'm planning to go ahead and push this patch - I'll give it a couple
> more days in case someone wants to shout stop!
>
> On input performance:
>
>   - I tested this and was a <1% slow down, which seem acceptable to
>     me,
>
>   - I notice that readline reads its input one character at a time
>     too, so now our non-readline input is handled the same way,
>
>   - This function is not used for reading commands from a file (I did
>     a simple test, and didn't hit this function), so shouldn't impact
>     that case at all.
>
> On output performance:
>
>   - The unbuffering will only impact the output file descriptor for
>     the new-ui case, usually, in all other cases, in and out are
>     separate file descriptors,
>
>   - The new-ui command only really makes sense for spinning up mi
>     interpreters,
>
>   - The mi interpreter buffers its output in string_files (see
>     mi/mi-out.c), and then writes the output in a single command, so
>     we shouldn't see any change in performance.
>
> As this patch fixes a real bug, I think, lets merge this now, and if
> there's any issues later, we can figure out what to do then.

I've now pushed this patch.

Thanks,
Andrew