On 09/07/2012 02:32 AM, Alexander Komyagin wrote: > OK. I agree. It sounds rather reasonable to avoid excess code complexity > and CPU consuming in order to gain performance for the common case.
I am very glad that we are in agreement here. Will you work on a patch to fix ConnOpener? > 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. Agreed: We should document what comm_connect_addr() does. However, does it provide any guarantees even _after_ select() notification? According to Stevens, the current code may still report that there is no problem when in fact there is one (and we will detect it later during I/O), right? Thank you, Alex. > 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. >> >
