Re: poll() blocked / packets not received ?

From: Nicolas Cannasse
Date: Mon Oct 20 2008 - 13:25:53 EST

David Schwartz a écrit :
At least I will have nice sleep tonight.

Note that this solved your symptom, not your problem. You actually have two

1) You rely on TCP to detect a lost connection even by a side that will
never transmit any data. TCP simply does not do this. If you are not trying
to send data, you are not assured that a lost connection will be detected.
(You either need a timeout, or you need to send or dribble some data,
depending on the protocl.)

2) You hold a lock on a shared resource while you wait for a reply over a
network. If this is a low-level "block and wait indefinitely" lock, this
will cause many threads to line up behind a slow/stuck thread. The right fix
depends on your circumstances, but you need to use a synchronization
primitive that is suitable. (You need to be able to use multiple connections
or defer operations without holding a thread.)

I agree with both points, but I can't modify the MySQL protocol to implement that.

For (1) I can't add the timeout since I have no way to differentiate between a lost connection and a request that takes time to execute. I'll maybe check if the protocol allow pings while waiting for the request result, but I'm not sure it does.

For (2) the shared resources is on the database side, not on the server side. It's the transaction that have some rows locked. I have no solution for that.

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