Re: [libmicrohttpd] [EXTERNAL] Re: New Features: Create response from an IOVEC and Thread-safe Adding Connections to an EPoll Internal Thread MHD_Daemon

[Earp, Damon N. (GSFC-619.0)[SCIENCE SYSTEMS AND APPLICATIONS INC]]

Thanks for the feedback, 

I'm not surprised by any of the gripes, most of this stuff has been done for our use case and not in a portable manner. As neither of us are adept at this code we might need some guidance. We missed the addition of the new connection list that was added in 0.9.72 and will look to use that instead of our now duplicate implementation. Anything to minimize the number of changes we make will be great. 

> Could you explain the new functionality added by 0.9.72-internal_queue_connection.patch?

Hopefully most of these changes are moot with 0.9.72's `new_connections_head`. Now that we are aware of our duplicate effort we might start from scratch on this with a fresh 0.9.72 and see if any of these changes are needed still.

• We implemented an `internal_queue_connection` which appends the new connection struct to our (duplicated) new connection linked list, then uses the itc interface to inform the internal thread. All while using a dedicated mutex. We followed the `internal_add_connection` signature and saved all the parameters so we could easily call `internal_add_connection` in our event thread.
• Instead of exposing a new `MHD_queue_connection` we updated `MHD_add_connection` to detect a daemon that was MHD_USE_INTERNAL_POLLING_THREAD and MHD_USE_EPOLL. In this case we use the `internal_queue_connection` which didn't race when called from our external thread.
• The changes at `@@ -4992,6 +5054,32 @@` is for `MHD_epoll`. This change is where we drain our connection queue and call `internal_add_connection` on from the internal thread.
• The changes around `daemon->listen_socket_in_epoll = true;` and `epoll_ctl` are in `setup_epoll_to_listen`. We had to reorder this logic because the itc eventfd wasn't being registered with epoll when MHD_USE_NO_LISTEN_SOCKET was paired with MHD_USE_INTERNAL_POLLING_THREAD and MHD_USE_EPOLL. My guess is we will run into this again and this might be the only change that remains in this patch.
  
• All other changes are just setting up our linked list struct, mutex, and header definitions

> The header sys/uio.h is not portable.

Noted, we will work on making the iovec code portable and emulating sendmsg on incompatible platforms. 

> Did you run any tests with/without your patch? Did it significantly increase throughput?

We have run correctness tests but not benchmarking, patched 0.9.62 has been in production for over a month using these changes. We used perf early on and noticed memcpys dominating our cpu time which is why we started down this path. We've had times where end users have requested > 5 TB of data in a sort timespan and there were 4 copies made going from kernel through our program and back to kernel. With the addition of a unit test we can compare MHD_create_response_from_iovec to MHD_create_response_from_callback. 

> Please, use "diff -p" next time as it simplify reviewing the changes.

On it, first time doing this.

> It would be great if you could update the libmicrohttpd manual

Will do.

> It would be good to have a simple unit test

We will look into adding one. 

Thanks,

Damon Earp

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From: Christian Grothoff <grothoff@gnunet.org>
Sent: Friday, January 8, 2021 13:47
To: libmicrohttpd development and user mailinglist <libmicrohttpd@gnu.org>
Cc: Earp, Damon N. (GSFC-619.0)[SCIENCE SYSTEMS AND APPLICATIONS INC] <damon.n.earp@nasa.gov>; Sebald, Lawrence (GSFC-619.0)[SCIENCE SYSTEMS AND APPLICATIONS INC] <lawrence.sebald@nasa.gov>
Subject: [EXTERNAL] Re: [libmicrohttpd] New Features: Create response from an IOVEC and Thread-safe Adding Connections to an EPoll Internal Thread MHD_Daemon

 

Dear Damon,
Dear Lawrence,

This is a good place for such patches.
Like Evgeny, I'm in principle in favor, but have also some "major" gripes:

1) It would be great if you could update the libmicrohttpd manual (in
docs/) to document the new external API function(s).  Just a few
sentences explaining the arguments and what the function does.

2) It would be good to have a simple unit test (see src/testcurl/ for
examples), especially so that we find out if basic things break on
certain platforms (given the non-portable nature of the patch).

Happy hacking!

Christian

On 1/8/21 6:14 PM, Earp, Damon N. (GSFC-619.0)[SCIENCE SYSTEMS AND
APPLICATIONS INC] via libmicrohttpd wrote:
> We've made changes to libmicrohttpd that we'd like to see integrated into the project. If this isn't the appropriate forum for submitting patches please let us know.