On Mon, 17 Dec 2012, Stefan Fritsch wrote:
On Sunday 16 December 2012, Niklas Edmundsson wrote:
I'm playing around with the event mpm in httpd 2.4.3, and I've come
to the conclusion that the stats reported by mod_status is rather
strange. I'm not sure if it's a bug or simply not implemented...
My test case is just a simple file transfer of a DVD image, no
mod_cache etc. involved.
I then, naively perhaps, expect that I should se this file transfer
on /server-status, but I see little to no trace of it.
I would at least expect it to consistently show up as writing
somewhere, but "writing" under "Async connections" mostly shows 0.
It should alternate between "Async connections/writing" and
"threads/busy". If not, then that's a bug.
Ah, it should alternate. I'll check this for this when I get round to
doing some more testing.
I get the underlying reason for this behaviour, but from the viewpoint
of a generic server admin without grasp of the underlying code it gets
slightly confusing when stuff bounces around like that.
I'll double check if I have missed some docco, and cobble up a
suggestion for improvement if I find that it's unclear/missing.
Another interesting thing is that the client ip shows up all over
the place in the server/thread list, but without any info on what
the client is doing. I'm guessing that this is the async
processing going on, but the status shown on /server-status is
just plain confusing.
Am I missing something?
Maybe a scoreboard update call is missing at some point. I will have
to look at it.
The fact that the client ip shows up on all threads points to some
potential optimization: Recently active threads should be preferred,
because their memory is more likely to be in the cpu caches. Right
now, the thread that has been idle the longest time will do the work.
Ah, virtually guaranteeing that the thread with the coldest cache gets
to do the work...
I definitely agree on the potential for improvement here, you would
most likely want to select either the thread that processed this
request last time, or the most recently active idle thread. These two
conditions kinda collides though, so the challenge is probably to come
up with some rather cheap selection algorithm that is good enough.
Sure, I can look in the server logs to find out what files have
been sent, but that's not quite the same as the "what is happening
right now" I'm used to /server-status presenting to me.
Is it possible to patch mod_status to show something useful for the
event mpm too, or is the information needed not available? For
example, I would expect to find a list of all the current requests
being processed somehow. As it currently stands it seems that all
replies currently being sent in async mode is a "black hole" on
/server-status.
The problem is that there is no fixed limit for the number of
connections being processed, which makes it hard to put that
information into a shared memory table. And without that, there is no
way for the process handling the server-info request to get the
information from the other processes. But I agree that this should be
fixed somehow.
Some kind of mechanism to extract the info when needed might be
preferred, as it would mean zero impact except when you're accessing
/server-status, but I don't know enough of the innards of httpd/event
to say if it's even possible.
An alternative could be to use the socache stuff to track this for
those that can take the performance impact (in our application,
large-file serving, it would be negligable impact for example).
/Nikke
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Niklas Edmundsson, Admin @ {acc,hpc2n}.umu.se | ni...@acc.umu.se
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