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Perfect is an open source server framework for Swift. It contains a built-in HTTP/S server and URI routing system and is primarily focused on building network APIs and REST services.

When Apple announced their intent to open source the Swift language and port it to Linux we were excited about the possibility of running Swift on the server and taking advantage, not only of Swift's strong type safety and clean syntax, but of the benefits of code sharing between clients and servers. Immediately after Apple's announcement (some several months before any actual release) we had embarked on bringing our experience with servers and server-side languages to the platform, resulting in Perfect.

Since then, we have continued to develop and mature Perfect and have been using it ourselves for internal and for client projects with good results.

Perfect is built upon our own general purpose networking library (written in Swift, of course) called Perfect-Net. Perfect-Net utilizes the OS level kqueue/epoll APIs (for macOS and Linux, respectively) and provides asynchronous accept, read, and write support, and serves as the basis for not only Perfect's HTTP server, but various other networking related code we and our users have written over the last few years.

SwiftNIO (henceforth referred to as NIO) is a relatively new open source project from Apple. Written in Swift, NIO itself provides asynchronous networking APIs as well as several concrete protocol implementations such as HTTP and WebSockets. NIO is (I'm told) modeled after Netty; a Java package with the same reason for being.

NIO's APIs would be considered more high level than Perfect-Net's. While Perfect-Net provides a simple system for installing a callback to be invoked when a network event occurs, NIO's system involves chaining handlers using a pipelining scheme, where each component handles a particular aspect of network protocol encoding, decoding, and framing.

While NIO is certainly interesting (any open source package produced and released by Apple would qualify as such), we were initially skeptical as to whether we could benefit from it. We have generally approached Perfect's core in a conservative manner, keeping APIs stable and only adding features which we knew could be done without disruption to existing code bases. Perfect-Net had been working quite well for us; it performed efficiently and with a negligible memory footprint. We wrote it and understood how it worked. Fixes and improvements could be added rapidly and with a good deal of assurance that the impacts would be minimal.

That said, there are certainly advantages in not having to maintain every aspect of a complex software project. Benefits come from being able to participate in a larger ecosystem, giving and receiving contributions from a much grander team than we alone could muster. Additionally, our users had begun to ask if and when we would be adopting NIO into Perfect. An answer of "we don't think it will be of benefit", without any evidence either way, is unsatisfactory. Finally, we had begun development of a new URI routing system which capitalized on newer additions to the Swift langage. This new system would be presented in a planned Perfect 4.0 release and so, if we were to adopt NIO, this would be the best time to do it. However, performance is very important for us. If moving to NIO was going to be a degradation to any degree then we would be hard pressed to spend the time migrating to it just because it was there.

We decided that if we were going to move to NIO we would do it in an informed manner. This would involve doing an initial implementation of Perfect running with our new routing system and with NIO at its core. We would then do a series of performance related tests pitting Perfect 3 against Perfect-NIO and see how they compared. We also wanted to run the same tests on several of the more popular HTTP frameworks in common usage today. The purpose of this was to help us gauge where Perfect (with or without NIO) currently stood in relation to the pack. If, during our testing, there was anything we could discover and change to improve our outcomes, then we would try and do so.

Luckily, we had a coinciding opportunity to work with the Centre of Excellence in Next Generation Networks (CENGN). CENGN provides programs and resources with the goal of helping Canadian small and medium sized information technology focused businesses to grow. CENGN was able to provide us with a closed networking environment and several pieces of hardware which would have been difficult for us to acquire on our own. We planned to use these resources to perform heavy stress testing of Perfect 3 and Perfect-NIO, as well as the other candidate frameworks we had identified.

Test Plan

The hardware provided to us was a pair of Cisco UCS C240 M5 rack servers. These each contained 36 Intel Xeon Gold 6140 CPUs running at 2.30GHz, with 384GB of RAM, and were connected with a 10G link. We figured this would suffice for our purposes.

One of the machines was split into four, 4 CPU VMs (the clients), and the other was left bare metal (the server). The server was where the server frameworks would run while the VMs would run JMeter to perform the tests. Ubuntu 16.04 was installed on all.

Of the four clients, one would be employed as a JMeter master while the other three would be JMeter minions. Zabbix with Postgres was installed on one of these VMs as well, and the Zabbix agent was placed on the server. The purpose of this was to be able to garner additional metrics from the server while the tests are being run. In the end we used Zabbix only to gather the ongoing CPU usage during the tests. Zabbix was configured to collect CPU usage once a second.

The server and all clients had their ephemeral port range widened, max open files increased, and tcp_tw_recycle and tcp_tw_reuse enabled.

All the frameworks we intended to test were then installed on the server. We settled on the following:

Django (2.1.2, Python 3.7, Apache+fcgi)
Go (1.11)
Kitura (2.5, Swift 4.2.1)
Laravel (5.7.9, PHP 7.1.19, Apache+mod_php)
Node (10.11, cluster mode)
Perfect (3, Swift 4.2.1)
Rails (5.2.1, Ruby 2.5.1, Apache+Passenger)
Spring MVC (2.0.5, Kotlin 1.2.51, Java 1.8)
Vapor (3.1, Swift 4.2.1)

Important notes:

  • The version of Vapor being tested was itself already running on NIO while Kitura was not (it's apparently an option that can be enabled, but we did not do that). This provided a great opportunity to compare performance of the various Swift frameworks while running on a few different networking cores.
  • Rails with Passenger had its minimum number of processes increased to 72 in order to match the number of available cores (with hyper-threading: 36x2). This Apache instance would be started and I would wait until all instances had launched and quieted down before commencing any test run. After completion that Apache instance would be halted.
  • Node was running in cluster mode, meaning a node process was launched for each available core, so 72 processes.

We then proceeded to add the test related endpoints to each of the frameworks. We settled on the following and implemented them for each framework and tested them individually to ensure they were operational and returning proper results.

	Empty request and response
	Empty request, 2048 byte response
	Empty request 32768 byte response
	Receive, decode and iterate through 26 search arguments, 2048 byte response
	Receive, decode and iterate through 26 url-encoded POST arguments, 2048 byte response
	Receive, decode and iterate through 26 multipart form encoded POST arguments, 2048 byte response
	Receive, decode and echo JSON post body data
	Randomly runs all POST/GET tests, 2048 & 8192 response, and JSON echo

All endpoint response data consisted of a series of 'A's repeated. Since we weren't concerned with comparing programming language performance per se, this response data was pre-calculated and served from a constant variable as best each language permitted. For the same reason, an attempt was also made to do as little work as possible within each URI handler.

We then created the JMeter test plan files (.jmx) for each endpoint and some shell scripts to help drive the test procedure. As described earlier, the clients consisted of one JMeter master and three minions. All actual traffic to the server would be coming from the three clients and all final result data would be sent to the master for aggregation. Most tests would generate up to a maximum of 9000 concurrent requests, ramping up in steps over a four minute period. The traffic level would then be held at peak for three minutes before the test was complete. Each test for each framework would take seven minutes to run, plus a trailing several minutes for the JMeter master to process and combine all client results into one log file in csv format.

In addition, an export of the collected CPU usage data would be copied to csv from the Zabbix related Postgres server. Only data corresponding to the individual test's run time would be exported.

The resulting log files from JMeter, from which all result data was gathered, could be quite large, up to several hundred GB each, depending on how many requests the respective server was able to handle. Because of this, the driving test scripts would zip up the result data and delete the raw csv files (a full run of all framework tests would easily fill up the available drive space without this step, which is generally sub-optimal). These final zip files would then be manually scp'd from the server to a local machine outside of the CENGN network for further processing.



Running an individual test produces two csv files. One contains the log file from JMeter, the other contains the CPU usage data gathered from Zabbix. A shell script would unzip the CPU data (which was ultimately much smaller than the JMeter data) to a temporary location and would streaming-unzip the JMeter data while piping it to a custom (Swift based) process. This process would normalize all the timestamp data from the source such that the first request occurred at zero seconds. This aided greatly in creating charts for the result data. It would also insert the matching CPU data for each span of seconds as an additional csv column and drop some columns which were not relevent to the final reporting process. And finally, it would output the resulting data to stdout in chunks as csv which was streamed into Postgres.

Each framework/test combination would have its data stored in its own table. This made working with the data much more efficient. The tests were run many, many times as the process was improved and as Perfect-NIO was developed and iterated on. Old test data was dropped for each new run.


Before a chart for any framework/test combination was generated, the data was exported from Postgres as csv. However, this data was first re-processed. This processing was done in Postgres itself as part of the select statement. Timestamp data was truncated to seconds (stripping off milliseconds) and all rows were grouped and ordered by this resulting time data. For each second, the request elapsed time, thread count, and latency were averaged. The elapsed request time's 25/50/75/95-th percentiles were calculated, and finaly the average cpu usage per second was calculated.

The above was only done for successful requests. Any failed requests were separately selected and written to their own file using the same seconds calculation and grouping.

The procedures were structured in this way so that data with the highest fidelity could be kept in Postgres while more relevant and smaller data sets could be fed to gnuplot for chart generation.


Once a framework/test combination's data was imported into and re-exported out of Postgres, it was fed into gnuplot. Gnuplot would then spit out a pdf containing three charts. These charts show:

  • Requests per second over time, including load level, and overall average requests per second.
  • CPU utilization over time, including load level.
  • Request elapsed time by 25/50/75/95-th percentile over time, including load level, and request errors, if any.

"Load level", above, refers to the number of requests to completed during any given second. CPU utilization is indicated by the shifting colour pallet; low to high, from purple to red. The maximum value is 7200.


Initial test results showed that Perfect-Net performed pretty well. Amongst the Swift based frameworks, it outperformed Vapor by a small margin, and Kitura by a large margin in many (but not all) of the tests. However, it fell well behind Node and Spring, which outperformed all other frameworks. Go fell slightly below Vapor. The remaining frameworks, Rails, Laravel, Django, all below that.

With regards to Perfect-NIO, we initially saw similar performance as Vapor, which makes sense because we were now running on the same networking core. But this also meant that Perfect-NIO was underperforming Perfect-Net. We also noticed that, by default, NIO based apps were not scaling vertically so as to take full advantage of the hardware we happened to be running on. It appeared as though the default accept queue system was not able to keep the worked queue full. This effect was not seen when running on more modest hardware. For example, testing on an 8 core MacBook Pro showed no anomolies and the machine appeared to be as busy as expected when under heavy server load.

After consultation with the SwiftNIO team we were able to devise a method for configuring and launching the NIO based server in a manner which was better able to take advantage of this hardware. An individual listener was launched for each CPU. The accepting sockets were all created with SO_REUSEPORT, which allowed them to all be bound to the same port. Each accept socket has its own worker queue with a capacity equal to the CPU count. While this scheme ended up using more resources at launch, it made a dramatic difference in performance under load. Compared to Perfect-Net, Perfect-NIO with this change was now 3x faster.

While SO_REUSEPORT and multiple acceptors worked well on Linux, the behaviour of SO_REUSEPORT on Linux and macOS differ and this change diminished performance on macOS. While we don't expect users to be deploying production servers on macOS, it is the primary development platform and we still wanted to have things perform there as well as possible. Therefore we made launching multiple acceptors an option in our API. Servers running on comperable hardware may find enabling this scheme to be beneficial.

Seeing that Perfect on NIO could perform very well compared to our existing offering, we then continued to test, analyze, and optimize Perfect-NIO. The results shown below are taken from our final complete run of all framework tests.



Empty request/response. Average requests per second:

45878 Spring
35834 Perfect-NIO
26877 Kitura
22855 Node
15738 Perfect-Net
15246 Vapor
14287 Go
10105 Rails
 7810 Laravel
 5493 Django


Empty request, 2048 byte response. Average requests per second:

46333 Perfect-NIO
46075 Spring
22726 Node
15579 Perfect-Net
14539 Vapor
14079 Go
10616 Rails
 8616 Laravel
 5599 Django
 2764 Kitura


  • Kitura's performance seemed to diminish based on how much data was returned. One can see similar performance in the getArgs2048, postArgs2048, and postArgsMulti2048 tests, and even further diminished performance in 32768. The reasons are unknown but I did verify it wasn't response compression being turned on.


Empty request, 32768 bytes response. Average requests per second:

32027 Perfect-NIO
30947 Spring
22461 Node
17411 Perfect-Net
13451 Vapor
13027 Go
 9197 Rails
 8467 Laravel
 5348 Django
  185 Kitura


Average requests per second:

29751 Perfect-NIO
29742 Spring
22699 Node
16105 Perfect-Net
12952 Go
11335 Rails
 8109 Laravel
 4687 Django
 2314 Kitura
  388 Vapor


Average requests per second:

39070 Spring
26634 Perfect-NIO
22449 Node
12036 Perfect-Net
10094 Rails
 8424 Laravel
 7735 Go
 4240 Django
 2512 Kitura
  419 Vapor


Average requests per second:

13461 Node
10732 Perfect-NIO
10621 Perfect-Net
10267 Rails
 9034 Laravel
 4844 Go
 4106 Spring
 2025 Kitura
 1509 Django
  311 Vapor


  • Both Perfect-Net and Perfect-NIO use the same mime parser.
  • Spring seemed to collapse on this task. It did quite well with url-encoded POST but not with multipart.


Average requests per second:

46007 Spring
23969 Perfect-Net
22828 Node
20257 Perfect-NIO
13980 Kitura
10775 Go
 8946 Rails
 8745 Vapor
 8373 Laravel
 5187 Django


  • Some frameworks try and enforce a string object encoding methodology on JSON data, others treat it as a dictionary. I tried to keep with the lowest friction idiom for the framework.


Average requests per second:

24053 Spring
22759 Perfect-NIO
21675 Node
19148 Perfect-Net
10334 Rails
 9881 Go
 8637 Laravel
 4084 Django
 3206 Kitura
  754 Vapor


  • Since this mix test cycled through the full test set, if a framework did particularly bad on one of them it ended up having a big impact on these results. You can see this in the Spring and Vapor result charts.

/2048 Heavy

Empty request, 2048 bytes response, 20k clients. Average requests per second:

40015 Perfect-NIO
39070 Spring
23445 Node
15495 Perfect-Net


  • This test cranked things up a little further from the base /2048 test. Only the top performing frameworks were included (plus Perfect-Net for reference).

Take Aways

Memory usage did not appear to be an issue with any framework. While the server test machine had beaucoup RAM, I didn't notice any leaks or unbounded growth in memory usage while tests were running.

Node performed consistently well. One can see a slight dip when it comes to parsing multipart form data (though it was still the top performer on that test), but otherwise it maintained at approximately 22k requests per second for every other test.

Go seemed to underperform my personal expectations; only utilizing a few of the many available CPUs. It's possible it would benefit from the same multi-acceptor methodology we applied to Perfect-NIO.

Subsequent Work

Finished Perfect-NIO. Completed WebSockets, compression (gzip, deflate), static file serving, and Mustache support.

Perfect 3 -> 4 compatability layer. Permits Perfect 3 code to run on Perfect-NIO with only changes to Package.swift file and imports.

Future Work

HTTP/2 support.

HTTP multiplexer.

Support for SwiftNIO 2 and Swift 5.

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