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SubjectRe: [net-next PATCH v4 2/3] net: TCP thin linear timeouts
On Thu, 18 Feb 2010, Andreas Petlund wrote:

> On 02/18/2010 10:09 AM, Ilpo Järvinen wrote:
> > On Thu, 18 Feb 2010, Franco Fichtner wrote:
> >
> >> Andreas Petlund wrote:
> >>> On 02/18/2010 09:41 AM, Ilpo Järvinen wrote:
> >>>
> >>>> On Wed, 17 Feb 2010, David Miller wrote:
> >>>>
> >>>>
> >>>>> From: Andreas Petlund <apetlund@simula.no>
> >>>>> Date: Tue, 16 Feb 2010 15:40:41 +0100
> >>>>>
> >>>>>
> >>>>>> @@ -341,6 +342,8 @@ struct tcp_sock {
> >>>>>> u16 advmss; /* Advertised MSS
> >>>>>> */
> >>>>>> u8 frto_counter; /* Number of new acks after RTO */
> >>>>>> u8 nonagle; /* Disable Nagle algorithm?
> >>>>>> */
> >>>>>> + u8 thin_lto : 1,/* Use linear timeouts for thin
> >>>>>> streams */
> >>>>>> + thin_undef : 7;
> >>>>>>
> >>>>> There is now a gap of 3 unused bytes here in this critical
> >>>>> core TCP socket data structure.
> >>>>>
> >>>>> Please either find a way to avoid this hole, or document
> >>>>> it with a comment.
> >>>>>
> >>>> There would be multiple bits free for use in both frto_counter and nonagle
> >>>> byte.
> >>>>
> >>>>
> >>>
> >>> I was playing aroud with this setup:
> >>>
> >>> =========
> >>> u8 nonagle : 4,/* Disable Nagle algorithm? */
> >>> thin_lto : 1,/* Use linear timeouts for thin streams */
> >>> thin_dupack : 1,/* Fast retransmit on first dupack */
> >>> thin_undef : 2;
> >>> =========
> >>>
> >>> Do you think that would do the trick?
> >>>
> >>
> >> According to Ilpo, it would be ok to reduce both ftro_counter and
> >> nonagle, so why not join all these into u16 and leave the remaining
> >> free bits documented for other people. Like this:
> >>
> >> u16 frto_counter:x; /* Number of new acks after RTO */
> >> u16 nonagle:y; /* Disable Nagle algorithm? */
> >> u16 thin_lto:1; /* Use linear timeouts for thin streams */
> >> u16 unused:15-x-y;
> >>
> >> Not sure about the y and x. Ilpo, can you comment on those values?
> >
> > I don't remember top of the hat how much of nonagle used, but for
> > frto_counter max value was 3 iirc.
>
> I think nonagle uses 4 bits:
> ======
> #define TCP_NAGLE_OFF 1 /* Nagle's algo is disabled */
> #define TCP_NAGLE_CORK 2 /* Socket is corked */
> #define TCP_NAGLE_PUSH 4 /* Cork is overridden for already queued data */
> ======
>
> > However, I'm unsure if compiler is
> > nowadays wise enough to handle bitfields in some not all so stupid way.
>
> Would you then recommend to use a byte for each value, thus avoiding the
> bitfields?

I don't know about the current compilers but at least in past there has
been a bias against bitfields. Alternative would be to combine and code
the accessors manually (thus bypassing any "too clever" compiler
intelligence).

--
i.
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