Stefan Beller <sbel...@google.com> writes:
>>> + while (1) {
>>> + ssize_t len = xread(cp->err, buf, sizeof(buf));
>>> + if (len < 0)
>>> + die("Read from child failed");
>>> + else if (len == 0)
>>> + break;
>>> + else {
>>> + strbuf_add(&out, buf, len);
>>> + }
>>
>> ... and the whole thing is accumulated in core???
>
> The pipes have a limit, so we need to empty them to prevent back-pressure?
Of course. But that does not lead to "we hold everything in core".
This side could choose to emit (under protection of args->mutex)
early, e.g. after reading a line, emit it to our standard output (or
our standard error).
> And because we want to have the output of one task at a time, we need to
> save it up until we can put out the whole output, no?
I do not necessarily agree, and I think I said that already:
http://thread.gmane.org/gmane.comp.version-control.git/276273/focus=276321
>>> + }
>>> + if (finish_command(cp))
>>> + die("command died with error");
>>> +
>>> + sem_wait(args->mutex);
>>> + fputs(out.buf, stderr);
>>> + sem_post(args->mutex);
>>
>> ... and emitted to standard error?
>>
>> I would have expected that the standard error would be left alone
>
> `git fetch` which may be a good candidate for such an operation
> provides progress on stderr, and we don't want to intermingle
> 2 different submodule fetch progress displays
> ("I need to work offline for a bit, so let me get all of the latest stuff,
> so I'll run `git submodule foreach -j 16 -- git fetch --all" though ideally
> we want to have `git fetch --recurse-submodules -j16` instead )
>
>> (i.e. letting warnings from multiple jobs to be mixed together
>> simply because everybody writes to the same file descriptor), while
>> the standard output would be line-buffered, perhaps captured by the
>> above loop and then emitted under mutex, or something.
>
>>
>> I think I said this earlier, but latency to the first output counts
>
> "to the first stderr"
> in this case?
I didn't mean "output==the standard output stream". As I said in
$gmane/276321, an early output, as an indication that we are doing
something, is important.
> Why would we want to unplug the task queue from somewhere else?
When you have a dispatcher more intelligent than a stupid FIFO, I
would imagine that you would want to be able to do this pattern,
especially when coming up with a task (not performing a task) takes
non-trivial amount of work:
prepare task queue and have N threads waiting on it;
plug the queue, i.e. tell threads that do not start picking
tasks out of it yet;
large enough loop to fill the queue to a reasonable size
while keeping the threads waiting;
unplug the queue. Now the threads can pick tasks from the
queue, but they have many to choose from, and a dispatcher
can do better than simple FIFO can take advantage of it;
keep filling the queue with more tasks, if necessary;
and finally, wait for everything to finish.
Without "plug/unplug" interface, you _could_ do the above by doing
something stupid like
prepare a task queue and have N threads waiting on it;
loop to find enough number of tasks but do not put them to
task queue, as FIFO will eat them one-by-one; instead hold
onto them in a custom data structure that is outside the
task queue system;
tight and quick loop to move them to the task queue;
keep finding more tasks and feed them to the task queue;
and finally, wait for everything to finish.
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