On the 2.4.x branch:

Changes with Apache 2.4.26

  *) mod_http2: better performance, eliminated need for nested locks and

     thread privates. Moving request setups from the main connection to the

     worker threads. Increase number of spare connections kept.

     [Stefan Eissing]

  *) mod_http2: input buffering and dynamic flow windows for increased 

     throughput. Requires nghttp2 >= v1.5.0 features. Announced at startup

     in mod_http2 INFO log as feature 'DWINS'. [Stefan Eissing]

  *) mod_http2: h2 workers with improved scalability for better scheduling

     performance. There are H2MaxWorkers threads created at start and the

     number is kept constant for now. [Stefan Eissing]

  *) mod_http2: obsoleted option H2SessionExtraFiles, will be ignored and

     just log a warning. [Stefan Eissing]

  *) mod_http2: fixed PR60869 by making h2 workers exit explicitly waking up

     all threads to exit in a defined way. [Stefan Eissing]

  *) core: Add %{REMOTE_PORT} to the expression parser. PR59938

     [Hank Ibell <hwibell gmail.com>]

     modules already shut down and slave connections still operating.

     [Stefan Eissing]

  *) mod_http2: stream timeouts now change to vhost values once the request

     is parsed and processing starts. Initial values are taken from base

     server or SNI host as before. [Stefan Eissing]

  *) mod_proxy_http2: fixed retry behaviour when frontend connection uses 

     http/1.1. [Stefan Eissing]

  *) mod_http2: separate mutex instances for each bucket beam, resulting in 

     less lock contention. input beams only created when necessary.

     [Stefan Eissing]

  *) mod_lua: Support for Lua 5.3

  *) mod_proxy_http2: support for ProxyPreserverHost directive. [Stefan Eissing]

  modules/http2/h2_session.c         modules/http2/h2_stream.c 

  modules/http2/h2_switch.c          modules/http2/h2_ngn_shed.c 

  modules/http2/h2_task.c            modules/http2/h2_util.c

  modules/http2/h2_worker.c          modules/http2/h2_workers.c

  modules/http2/h2_workers.c

)

SET(mod_ldap_extra_defines           LDAP_DECLARE_EXPORT)

SET(mod_ldap_extra_libs              wldap32)

            the connection will buffer this amount of data and then suspend the

            H2Worker.

        </p>

        <p>

            If you serve a lot of static files, <directive module="mod_http2">H2SessionExtraFiles</directive>

            is of interest. This tells the server how many file handles per

            HTTP/2 connection it is allowed to waste for better performance. Because

            when a request produces a static file as the response, the file handle

            gets passed around and is buffered and not the file contents. That allows

            to serve many large files without wasting memory or copying data 

            unnecessarily. However file handles are a limited resource for a process,

            and if too many are used this way, requests may fail under load as

            the amount of open handles has been exceeded.

        </p>

    </section>

    <section id="misdirected"><title>Multiple Hosts and Misdirected Requests</title>

        </usage>

    </directivesynopsis>

    <directivesynopsis>

        <name>H2SessionExtraFiles</name>

        <description>Number of Extra File Handles</description>

        <syntax>H2SessionExtraFiles <em>n</em></syntax>

        <contextlist>

            <context>server config</context>

            <context>virtual host</context>

        </contextlist>

        <usage>

            <p>

                This directive sets maximum number of <em>extra</em> file handles

                a HTTP/2 session is allowed to use. A file handle is counted as

                <em>extra</em> when it is transferred from a h2 worker thread to

                the main HTTP/2 connection handling. This commonly happens when

                serving static files.

            </p><p>

                Depending on the processing model configured on the server, the

                number of connections times number of active streams may exceed

                the number of file handles for the process. On the other hand,

                converting every file into memory bytes early results in too 

                many buffer writes. This option helps to mitigate that.

            </p><p>

                The number of file handles used by a server process is then in

                the order of:

            </p>

            <pre>

                (h2_connections * extra_files) + (h2_max_worker)

            </pre>

            <example><title>Example</title>

                <highlight language="config">

H2SessionExtraFiles 10

                </highlight>

            </example>

            <p>

                If nothing is configured, the module tries to make a conservative

                guess how many files are safe to use. This depends largely on the 

                MPM chosen.

            </p>

        </usage>

    </directivesynopsis>

    <directivesynopsis>

        <name>H2SerializeHeaders</name>

        <description>Serialize Request/Response Processing Switch</description>

	$(OBJDIR)/h2_switch.o \

	$(OBJDIR)/h2_task.o \

	$(OBJDIR)/h2_util.o \

	$(OBJDIR)/h2_worker.o \

	$(OBJDIR)/h2_workers.o \

	$(OBJDIR)/mod_http2.o \

	$(EOLIST)

h2_switch.lo dnl

h2_task.lo dnl

h2_util.lo dnl

h2_worker.lo dnl

h2_workers.lo dnl

"

dnl # nghttp2 >= 1.14.0: invalid header callback

      AC_CHECK_FUNCS([nghttp2_session_callbacks_set_on_invalid_header_callback], 

        [APR_ADDTO(MOD_CPPFLAGS, ["-DH2_NG2_INVALID_HEADER_CB"])], [])

dnl # nghttp2 >= 1.15.0: get/set stream window sizes

      AC_CHECK_FUNCS([nghttp2_session_get_stream_local_window_size], 

        [APR_ADDTO(MOD_CPPFLAGS, ["-DH2_NG2_LOCAL_WIN_SIZE"])], [])

    else

      AC_MSG_WARN([nghttp2 version is too old])

    fi