[Haskell-cafe] Can a GC delay TCP connection formation?

Jason Dagit dagitj at gmail.com
Tue Nov 27 20:17:47 CET 2012

On Tue, Nov 27, 2012 at 11:02 AM, Jeff Shaw <shawjef3 at gmail.com> wrote:

> Hello Timothy and others,
> One of my clients hosts their HTTP clients in an Amazon cloud, so even
> when they turn on persistent HTTP connections, they use many connections.
> Usually they only end up sending one HTTP request per TCP connection. My
> specific problem is that they want a response in 120 ms or so, and at times
> they are unable to complete a TCP connection in that amount of time. I'm
> looking at on the order of 100 TCP connections per second, and on the order
> of 1000 HTTP requests per second (other clients do benefit from persistent
> HTTP connections).
> Once each minute, a thread of my program updates a global state, stored in
> an IORef, and updated with atomicModifyIORef', based on query results via
> HDBC-obdc. The query results are strict, and atomicModifyIORef' should
> receive the updated state already evaluated. I reduced the amount of time
> that query took from tens of seconds to just a couple, and for some reason
> that reduced the proportion of TCP timeouts drastically. The approximate
> before and after TCP timeout proportions are 15% and 5%. I'm not sure why
> this reduction in timeouts resulted from the query time improving, but this
> discovery has me on the task of removing all database code from the main
> program and into a cron job. My best guess is that HDBC-odbc somehow
> disrupts other communications while it waits for the DB server to respond.

Have you read section 8.4.2 of the ghc user guide?

Based on that I would check the FFI imports in your database library. In
the best case (-threaded, 'safe', and thread-safe odbc), I think you'll
find that N of these can run concurrently, but here your number of requests
is likely to be much greater than N (where N is the number of threads the
RTS created with +RTS -N).

I'm not sure how to solve your problem, but perhaps this information can
help you pinpoint the problem.

Good luck,
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