Subject: Re: [Bitcoin-development] bitcoind stops responding

From: Jeff Garzik <[email-masked]>
Date: Fri, 4 Oct 2013 11:05:29 -0400

On Fri, Oct 4, 2013 at 2:22 AM, Gavin Andresen <[email-masked]> wrote:
> RE: running into the maximum-of-4-keepalive-requests : simple workaround is
> to run with -rpcthreads=11 (or however many keepalive connections you need
> to support).  I agree that the rpc code should be smarter; making the last
> rpc thread ignore keepalive and always disconnecting should be a fairly
> simple patch, and "patches welcome."

It's all still working around a problem unchanged since Satoshi wrote
it:  the HTTP server code paths use blocking I/O.

Amusingly, we do this through an async I/O library, which helps
facilitate SSL, but all our connections and operations are blocking.

That's why RPC was multi-threaded in part:  to work around the ugly
blocking nature of the code.  At least with multiple threads, one
thread will not stall another even if the network stalls (or a
software bug triggers a stall etc.)

Hopefully I can dive into the code and make is truly async I/O.  It
takes some work, and I'm happy if someone else steals the task, but
that's what really needs to be done.

I tried the multi-threaded approach, writing an entire boost::asio
skeleton JSON-RPC HTTP server: https://github.com/jgarzik/rpcsrv
This is working, tested code that uses boost::asio properly.  It's
also quite a bit of LOC, and a bit messy to boot (four LOC per boost
async action and incomprehensible compiler errors in return for proper
async+MT).

A single thread with async I/O is likely sufficient for even heavy
uses of RPC -- since today it all goes through a big lock anyway.

-- 
Jeff Garzik
Senior Software Engineer and open source evangelist
BitPay, Inc.      https://bitpay.com/