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

Neil Davies semanticphilosopher at gmail.com
Wed Nov 28 10:21:39 CET 2012


Are you certain that all the delay can be laid at the GHC runtime? 

How much of the end-to-end delay budget is being allocated to you? I recently moved a static website from a 10-year old server in telehouse into AWS in Ireland and watched the access time (HTTP GET to check time on top index page) increase by 150ms.


On 27 Nov 2012, at 19:02, 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.
> To respond to Ertugrul, I'm compiling with -threaded, and running with +RTS -N.
> I hope this helps describe my problem. I c an probably come up with some hard information if requested, E.G. threadscope.
> Jeff
> On 11/27/2012 10:55 AM, timothyhobbs at seznam.cz wrote:
>> Could you give us more info on what your constraints are?  Is it necessary that you have a certain number of connections per second, or is it necessary that the connection results very quickly after some other message is received?
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