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From: Stefan Eissing <stefan.eissing@greenbytes.de>
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Subject: buckets across threads - question
Message-Id: <11EC3B28-0C00-442E-9B34-2DC010604068@greenbytes.de>
Date: Tue, 17 Mar 2015 16:59:43 +0100
To: dev@httpd.apache.org
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Hi,

I have a question regarding my http2 module implementation, where I seem =
to fail to wrap my head around the limitations of pools and bucket =
brigades in a multi-threaded environment. As I understand it, httpd has =
- so far - the underlying assumption:

 1 connection -> 1 request -> 1 thread

at a single point in time. This used to be quite static, as in =
mpm_worker, and was made more dynamic with mpm_event, which may switch =
the thread around. However even though entities might be exchanged, it =
is always 1-1-1 at a single point in time.

Now, as most of you are aware, with the HTTP/2 processing model, this =
holds no longer true. The relation there is 1-m-m* and, worse, the =
connection is multiplexing requests, so there are many active at the =
same time and their data will be interleaved. The request/thread =
coupling can stay as it is, but the connection relation is different.

I today have a working implementation and it has major drawbacks (among =
all the bugs I need to find):

1. it mimicks httpd objects that are not really there, e.g. =
pseudo-connections, so that there seems to be a 1-1 with connections and =
requests again (just as mod_spdy does). Therefore it needs to fake =
HTTP/1.1 requests, so that core filter can parse them, etc.

2. beautiful apr infrastructure like bucket brigades are not used when =
data needs to be passed between threads.

I have made an attempt today to make bucket brigades work for passing =
data between threads, but things keep crashing on me when load grows. =
Which is all my fault for not understanding these things fully. So, am I =
on a fool's errand? Can I have brigade A in thread 1 and brigade B in =
thread 2 and pass buckets between them without excessive copying? I =
think I am doing it wrong and the cleanups of the allocators run in the =
wrong thread in the end which is not healthy.

As for the request processing, I saw that there is a =
ap_process_request(r) to be had. Is it worthwhile to see if that can be =
called directly without ap_read_request() creating it. That would =
eliminated at least the request->HTTP/1 serialization...

Any ideas or hints appreciated.


//Stefan


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