Tickets :: Comment on [#13231] Hashtable session handler doesn't unlock on session close

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Comment	>>> This doesn't help. As I noted above, 10 requests/second is already >>> fairly heavy usage. Your solution bumps that up to 20 >>> requests/second (you have to assume worst-case scenario). >> >> No, you don't. The requests will follow a Poisson distribution, >> assuming mt_rand is not biased towards certain values > > It is. (although not for the numbers you gave). > >> So the with >> increasing number of requests, the average will quickly stabilise >> towards 10 requests/second. > > Not really. 10-20 requests is going to show bias one way or the > other. Sure enough, running a sample size of 20 mt_rand() calls > created an average time of 82806ns. 18% from the expected "median" > is not "stabilizing" in that sample size. And if we are running > thousands/millions of retries, the problem is not in the random > distribution but rather in the session access in the first place. > > And the original problem remains: you are potentially delaying the > normal scenario - a single request is queued - in an attempt to > make the outlier work better - since you potentially increase this > common case to 50% slower than it currently is. We need to optimize > for the normal scenario. I'm going to commit a slightly better > algorithm for this purpose, by ramping up to the the 0.1 second delay. > >>> And there is a general (albeit not mandatory) requirement that >>> requests SHOULD be handled in a FIFO basis. If Kronolith for some >>> reason sends 15 listEvents requests in a row, it is a reasonable >>> assumption that the 15th is the least important request. It should >>> not, by virtue of random luck, suddenly jump to 2nd in the queue. >> >> There is no way (whatever method you use) if these requests are >> handled by separate processes and it is required to get a lock on a >> shared resource >> If that is needed, a new request can only be >> initiated when the previous one is know to be active (not waiting to >> get a lock on a resource). Since this is not done, requests can (and >> will be) served in random order. > > Only for the memcache sessionhandler - or at least a poll-based > locking handler instead of an event-based handler. WHICH IS EXPECTED > (and one of the reasons why we don't recommend memcache > sessionhandler). That doesn't mean the assumption doesn't hold ... > and the fact that the other sessionhandlers do support this > generalization. > > Randomizing the queue order is something you do to prevent deadlocks. > It doesn't do anything to make a single access any faster. In some > instances it might be faster than the current strategy, but in some > instances it might be worse. > > The limiting factor remains the median polling time. > > I'm with Mike in that there definitely is no bug here - this is an > enhancement request at best. It's possible that the 0.1 sec poll > time is not ideal - but I'm not going to change that without evidence > to prove a better value, especially since the sessionhandler works > and the performance issues are known and have been known since the > beginning.
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