htcat

htcat is a utility to perform parallel, pipelined execution of a single HTTP GET. htcat is intended for the purpose of incantations like:

htcat https://host.net/file.tar.gz | tar -zx

It is tuned (and only really useful) for faster interconnects:

$ htcat http://test.com/file | pv -a > /dev/null
[ 109MB/s]

This is on a gigabit network, between an AWS EC2 instance and S3. This represents 91% use of the theoretical maximum of gigabit (119.2 MiB/s).

Installation

This program depends on Go 1.1 or later. One can use go get to download and compile it from source:

$ go get github.com/htcat/htcat/cmd/htcat

Help and Reporting Bugs

For correspondence of all sorts, write to [email protected]. Bugs can be filed at htcat's GitHub Issues page.

Approach

htcat works by determining the size of the Content-Length of the URL passed, and then partitioning the work into a series of GETs that use the Range header in the request, with the notable exception of the first issued GET, which has no Range header and is used to both start the transfer and attempt to determine the size of the URL.

Unlike most programs that do similar Range-based splitting, the requests that are performed in parallel are limited to some bytes ahead of the data emitted so far instead of splitting the entire byte stream evenly. The purpose of this is to emit those bytes as soon as reasonably possible, so that pipelined execution of another tool can, too, proceed in parallel.

These requests may complete slightly out of order, and are held in reserve until contiguous bytes can be emitted by a defragmentation routine, that catenates together the complete, consecutive payloads in memory for emission.

Tweaking the number of simultaneous transfers and the size of each GET makes a trade-off between latency to fill the output pipeline, memory usage, and churn in requests and connections and incurring their associated start-up costs.

If htcat's peer on the server side processes Range requests more slowly than regular GET without a Range header, then, htcat's performance can suffer relative to a simpler, single-stream GET.

Numbers

These are measurements falling well short of real benchmarks that are intended to give a rough sense of the performance improvements that may be useful to you. These were taken via an AWS EC2 instance connecting to S3, and there is definitely some variation in runs, sometimes very significant, especially at the higher speeds.

Tool	TLS	Rate
htcat	no	109 MB/s
curl	no	36 MB/s
aria2c -x5	no	113 MB/s
htcat	yes	59 MB/s
curl	yes	5 MB/s
aria2c -x5	yes	17 MB/s

On somewhat small files, the situation changes: htcat chooses smaller parts, as to still get some parallelism.

Below are results while performing a 13MB transfer from S3 (Seattle) to an EC2 instance in Virginia. Notably, TLS being on or off did not seem to matter, perhaps in this case it was not a bottleneck.

Tool	Time
curl	5.20s
curl	7.75s
curl	6.36s
htcat	2.69s
htcat	2.50s
htcat	3.25s

Results while performing a transfer of the same 13MB file from S3 to EC2, but all within Virginia:

Tool	TLS	Time
curl	no	0.29s
curl	no	0.75s
curl	no	0.44s
htcat	no	0.30s
htcat	no	0.30s
htcat	no	0.48s
curl	yes	2.69s
curl	yes	2.69s
curl	yes	2.62s
htcat	yes	1.37s
htcat	yes	0.45s
htcat	yes	0.59s

Results while performing a 4.6MB transfer on a fast (same-region) link. This file is small enough that htcat disables multi-request parallelism. Given that, it's unclear why htcat performs markedly better on the TLS tests than curl.

Tool	TLS	Time
curl	no	0.14s
curl	no	0.13s
curl	no	0.14s
htcat	no	0.23s
htcat	no	0.16s
htcat	no	0.17s
curl	yes	0.95s
curl	yes	0.97s
curl	yes	0.99s
htcat	yes	0.38s
htcat	yes	0.34s
htcat	yes	0.24s