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

Jason Dagit dagitj at gmail.com
Tue Nov 27 20:18:17 CET 2012

On Tue, Nov 27, 2012 at 11:17 AM, Jason Dagit <dagitj at gmail.com> wrote:

> 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?
> http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/ffi-ghc.html
Ahem, I meant *8.2.4*.
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