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This article explains one technique to improve the performace of the simple-sendfile package on which Warp relies. Readers are supposed to read "Improving the performance of Warp".

Before Andreas suggested me that the open()/close() system calls are a performance bottleneck, I found another bottleneck in the simple-sendfile package. Before talking how to avoid open()/close(), I would like to start with how to fix the latter bottleneck. I believe that this is a good introduction to understanding the architecture of Warp.

Warp is an HTTP engine for WAI (Web Application Interface). It runs WAI applications over HTTP. The type of WAI applications is as follows:

type Application = Request -> ResourceT IO Response

That is, an application takes Request and returns Response . As you can guess, Warp first receives an HTTP request from a client and parses it to Request . Then, Warp gives the Request to an application and takes a Response from it. Finally, Warp builds an HTTP response based on Response and sends it back to the client.

As I pointed out in "Mighttpd – a High Performance Web Server in Haskell", "system calls are evil for network programming in Haskell". So, Warp is defined to use as few system calls as possible.

In the HTTP request parser, Warp uses Network.Socket.ByteString.recv which issues the recv() system call. If a client uses HTTP pipelining, multiple HTTP requests can be received with one recv().

In the HTTP response builder, the situation is complicated because Response has three constructors:

ResponseBuilder Status ResponseHeaders Builder ResponseSource Status ResponseHeaders (Source (ResourceT IO) (Flush Builder)) ResponseFile Status ResponseHeaders FilePath (Maybe FilePart)

For ResponseBuilder and ResponseSource , Network.Socket.ByteString.sendAll - based on the send() system call- is used to send both an HTTP response header and body with a fixed buffer.

For ResponseFile , the old Warp used Network.Socket.ByteString.sendMany to send an HTTP response header and Network.Sendfile.sendfile to send a file as an HTTP response body. They are based on the writev() and sendfile() system call, respectively. The combination of writev() and sendfile() had a problem.

When I measured the performance of Warp, I always did it with high concurrency. That is, I always make multiple connections at the same time. It gave me a good result. However, when I set the number of concurrency to 1, I found Warp is really really slow on Linux and FreeBSD.

I realized that this is because Warp uses the combination of writev() and sendfile(). In this case, an HTTP header and body are sent in separate TCP packets. To send them in a single TCP packet (when possible), I implemented a new function called Network.Sendfile.sendfileWithHeader , which is available in simple-sendfile v0.2.4 or later.

On Linux, it uses the send() system call with the MSG_MORE flag to store a header and the sendfile() system call to send both the stored header and a file. On FreeBSD, it uses the sendfile() system call with the header arguments to send both a header and a file. This trick ensures that both a header and a body is sent in a single TCP packet (if the file is small enough).

This makes Warp at least 100 times faster when measured with httperf .