Re: ICAP connections under heavy loads

OK. I agree. It sounds rather reasonable to avoid excess code complexity
and CPU consuming in order to gain performance for the common case.

However, as I stated earlier, the comm.cc problem (actually semantics
problem) persists. I think it should be documented that second and
subsequent calls to comm_connect_addr() do not guarantee connection
establishment unless there was a correct select() notification.

On Thu, 2012-09-06 at 11:21 -0600, Alex Rousskov wrote:
> On 09/06/2012 02:35 AM, Alexander Komyagin wrote:
> > On Wed, 2012-09-05 at 09:59 -0600, Alex Rousskov wrote:
> >> On 09/05/2012 09:27 AM, Alexander Komyagin wrote:
> >>
> >>> So you think that it's ok for comm_coonect_addr() to return COMM_OK if
> >>> it was called before the appropriate select() notification. Am I right?
> >>
> >> Hard to say for sure since comm_coonect_addr() lacks an API description,
> >> and there are at least three similar but different ways the function is
> >> being used by Squid.
> >>
> >> One natural way to define this API is to say that it should return
> >> COMM_OK if and only if the socket is connected (making any select
> >> notifications irrelevant). However, this definition may be too
> >> CPU-expensive and/or too unportable to support. And this level of
> >> certainty may not actually be needed for current Squid needs!
> >
> > Maybe you're right, Alex. However, I would prefer this function to have
> > the very strict semantics: it should return COMM_OK if and only if the
> > socket is connected (just like you said). Because this way any
> > upper-level code can rely on it.
>
> That would be my preference as well, but I would like to see the costs
> of doing that first. If strict semantics is not currently needed and is
> more CPU/portability-expensive than the current code, then we should not
> perfect the code (beyond documentation). But again and again, I
> recommend resolving the higher-level (ConnOpener) issue before
> discussing any of this low-level stuff.
>
>
> >> I would not be surprised if there is some gray area where we cannot
> >> really tell for sure (without too much additional overhead or
> >> portability risk) whether the async socket is connected. The Stevens
> >> book seem to imply that much. Inside that gray area, the function
> >> should probably return COMM_OK so that the rest of the code works: If we
> >> guessed wrong, we will get failures during I/O, but the code has to deal
> >> with those anyway.
> >
> > I guess those I/O failures would cost us CPU cycles
>
> I/O failures are rare exceptions. We need to optimize the common case or
> at least not make it worse. The common case does not deal with timeouts
> and errors.
>
>
> > and make connection problems very-very-very hard to debug.
>
> Would not the error be the same, regardless of whether it is discovered
> during "connect" time or during "write" time?
>
>
> > Also all connection timeouts
> > become useless if we can't tell for sure whether the socket is really
> > connected.
>
> Why would they become useless? If Squid fails to connect (from Squid
> point of view!) in X seconds, it should treat this as a timeout and
> proceed accordingly. There will be vary rare events where Squid point of
> view would differ from OS point of view, but I do not see (a) why we
> should care about those very rare events and (b) how we can avoid them
> completely without implementing Squid inside the kernel.
>
>
>
> >>
> >> In other words, if you want to work on this, consider defining the API
> >> based on current Squid needs and then make sure we support those minimum
> >> requirements, keeping overheads and portability in mind. However, I
> >> would _start_ by fixing the calling code first (as it may affect the
> >> minimum requirements) -- your ConnOpener patch was a step in that direction.
> >
> > Amos wrote that calling code is actually OK.
>
> My interpretation is that Amos had explained what the code is doing. I
> did not hear Amos being convinced that the code does what it should. And
> I think that the code does not do what it should.
>
>
> Cheers,
>
> Alex.
>

-- 
Best wishes,
Alexander Komyagin