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
