[krbdev.mit.edu #6339] Fwd: krb5_sendauth vs NAGLE vs DelayedAck

[Ken Raeburn via RT]

Create a krb5int_net_writev helper function, calling writev in a loop  
like krb5_net_write.  Use it in krb5_write_message.

Begin forwarded message:

> From: John Hascall <john at iastate.edu>
> Date: January 14, 2009 15:22:34 EST
> To: kerberos at MIT.EDU
> Subject: krb5_sendauth vs NAGLE vs DelayedAck
> X-Spam-Score: 0
>
>
> I don't recall having seen this discussed on this
> list and google doesn't seem to either, so...
>
> I just discovered that hard way that the way that
> krb5_sendauth/krb5_recvauth work tickles the nasty
> interaction between the TCP NAGLE and DelayedAck
> features which inserts two ~200ms delays into the
> process:
>
> 16994 accessd 1231948663.402557 CALL  read(6,0xbfbffa54,4)
> 16994 accessd 1231948663.402561 GIO   fd 6 read 4 bytes
>       "\0\0\0\^S"
> 16994 accessd 1231948663.402567 CALL  read(6,0xa1e0520,0x13)
> 16994 accessd 1231948663.592835 GIO   fd 6 read 19 bytes
>       "KRB5_SENDAUTH_V1.0\0"
> 16994 accessd 1231948663.592850 CALL  read(6,0xbfbff914,4)
> 16994 accessd 1231948663.592854 GIO   fd 6 read 4 bytes
>       "\0\0\0\b"
> 16994 accessd 1231948663.592860 CALL  read(6,0xa1e2530,8)
> 16994 accessd 1231948663.592863 GIO   fd 6 read 8 bytes
>       "PV 1.00\0"
> 16994 accessd 1231948663.592868 CALL  write(6,0xbfbff99b,1)
> 16994 accessd 1231948663.592874 GIO   fd 6 wrote 1 bytes
>       "\0"
> 16994 accessd 1231948663.592879 CALL  read(6,0xbfbff914,4)
> 16994 accessd 1231948663.593675 GIO   fd 6 read 4 bytes
>       "\0\0\^A\M^U"
> 16994 accessd 1231948663.593681 CALL  read(6,0xa165800,0x195)
> 16994 accessd 1231948663.792905 GIO   fd 6 read 405 bytes
>       "...a bunch of binary goop..."
>
> This was pretty annoying when that 4/10s of a second was
> pretty much all of the connection lifetime.
>
> My guess is that this occurs anywhere that the kerberos
> libs use krb5_write_message().
>
> My solution was just to do:
>  int on = 1;
>  setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &on, sizeof(on));
> before calling krb5_sendauth() but a "better" approach might
> be for krb5_write_message to end up calling writev so it
> does one write instead of two, I think.
>
>
> John
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