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From "Brian J. France" <br...@brianfrance.com>
Subject Re: apr_socket_opt_set always sets TCP_DEFER_ACCEPT to 1
Date Tue, 22 Apr 2014 13:43:02 GMT

On Apr 22, 2014, at 8:53 AM, Jeff Trawick <trawick@gmail.com> wrote:
> 
>> On Mon, Apr 21, 2014 at 1:17 PM, Brian J. France <brian@brianfrance.com> wrote:
>> Just ran into a issue where httpd can only set TCP_DEFER_ACCEPT to 1.
>> 
>> apr_socket_opt_set will always set the option value to either 0/1, so even adding
a directive in httpd to allow tweaking the value it can't get it to the raw setsockopt call
via apr functions.
>> 
>> Right now we rebuilt our own version of apr with a default of 60, but want to work
on getting a patch to allow getting a httpd directive.
>> 
>> Thoughts on how what to tweak in apr?  Different apr_socket_opt_set type function
for TCP_DEFER_ACCEPT?
>> 
>> Brian
> 
> apr_socket_opt_set() can theoretically handle non-flag values; see APR_SO_SNDBUF/APR_SO_RCVBUF
for examples.


Doh, this is my bad:  on != one

http://svn.apache.org/viewvc/apr/apr/trunk/network_io/unix/sockopt.c?view=markup#l113

For some reason I completely missed that.  APR is fine, but httpd needs updated as it is hard
coded to 1:

  rv = apr_socket_opt_set(s, APR_TCP_DEFER_ACCEPT, 1);

Will move to the httpd list.


> Here's my suggestion:
> 
> * Fix the API (code+doc) in APR trunk (future 2.x) to respect the exact value of the
"on" parameter, as with APR_SO_SNDBUF/APR_SO_RCVBUF.
> * Applications using APR 1.x that need to set a value other than 1: Use apr_os_sock_get()+setsockopt()
> * If you are so motivated: Add non-default compile flag for APR 1.x that causes the value
passed via apr_socket_opt_set(APR_TCP_DEFER_ACCEPT) to be respected.
> 
> We occasionally correct broken/limited API behavior that could conceivably hurt some
unknown piece of code somewhere on a minor version change, but it seems simple enough for
the APR 1.x app to handle directly rather than prereq a certain APR version just for this
minor tweak.
> 
> --/--
> 
> What is the big picture?  My Linux tcp man page says:
> 
> TCP_DEFER_ACCEPT (since Linux 2.4)
>               Allow a listener to be awakened only when data arrives on the socket. 
Takes an integer value (seconds), this can bound the
>               maximum number of attempts TCP will make to complete the connection.  This
option should not be used in code intended to be
>               portable.
> 
> I find that very unclear.  "ACCEPT" and "listener...awakened" implies something with
the httpd/kernel interaction, and "complete the connection" is peer/peer interaction.
> 
> What happens if the 1 second timeout set now when setting APR_TCP_DEFER_ACCEPT via APR
expires?


While the man page says it is seconds, it is not a true seconds value as it is translated
into "maximum number of attempts TCP".  

So something like 10 seconds is 3-4 attempts, which could take longer than 10 seconds.



>  Drop connection without waking up server?


This is the behavior up until RHEL 6.x.


> Wake up server which then has to wait for data?


This is what RHEL 6 is doing and whey we noticed the issue.  Our kernel team is looking into
what changed between RHEL 5 and RHEL 6 (even between 6.2 to 6.3 is different).

Cheers,

Brian



> If it is "drop connection without waking up server" then IMO we need to really fix APR
1.5.x in the same manner that I suggested for trunk.
> 
> If it is "wake up server which then has to wait for data" then I wonder what you really
want.  No data came for 30 or 60 (or whatever) seconds then the server has to reach its own
timeout before we get rid of the connection?  Why wait so long?  The hard-coded 1 second timeout
doesn't seem so bad here (though in the fullness of time the app should choose the timeout).
 The optimization for normal scenarios is working, and httpd+kernel both get involved to clean
up basket case connections.
> 
> Maybe TCP_DEFER_ACCEPT does something else.
> 
> I could test and see, but I'm ASSuming that you already know :)  TIA for that!
> 
> -- 
> Born in Roswell... married an alien...
> http://emptyhammock.com/
> http://edjective.org/
> 


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