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I have heard a few people argue that when it comes to performance critical code you should prefer arrays over other collections (such as F#’s lists) as it benefits from sequential reads (which is faster than seeks) and offers better memory locality.
To test that theory somewhat, I wanted to see if there is any difference in how fast you can iterate through an array versus a list in F#, and how much faster you can map over an array compared to a list:
The result is a little surprising, whilst I wasn’t expecting there to be a massive difference in the iterating through the two types of collections, I didn’t think mapping over a list would be quite as slow in comparison. I knew that constructing a list is much heavier than constructing an array, but I didn’t think it’d take 22x as long in this case.
What was even more surprising was how much slower the Seq.iter and Seq.map functions are compared to the Array and List module equivalents! This is, according to John Palmer:
Once you call in to
Seqyou lose the type information – moving to the next element in the list requires a call to
IEnumerator.MoveNext. Compare to for
Arrayyou just increment an index and for
Listyou can just dereference a pointer. Essentially, you are getting an extra function call for each element in the list.
The conversions back to
Arrayalso slow the code down for similar reasons
As a work around, you COULD shadow the Seq module with iter and map functions that adds simple type checking and in the case of an array or list simply call the corresponding function in the Array or List module instead:
Whilst this approach will work to a certain extend, you should be careful with which functions you shadow. For instance, it’s not safe to shadow Seq.map because it can be used in conjunction with other functions such as Seq.takeWhile or Seq.take. In the base implementation, a line of code such as:
arr |> Seq.map incr |> Seq.take 3
will not map over every element in the source array.
With the shadowed version (see above) of Seq.map, however, this would first create a new array by applying the mapper function against every element in the source array before discarding all but the first three elements in the new array. This, as you can imagine, is far less efficient and requires much more memory space (for the new array) and defeats the purpose of using Seq module functions in most cases.
Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.
Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.
Skill up your serverless game with this hands-on workshop.
My 4-week Production-Ready Serverless online workshop is back!
This course takes you through building a production-ready serverless web application from testing, deployment, security, all the way through to observability. The motivation for this course is to give you hands-on experience building something with serverless technologies while giving you a broader view of the challenges you will face as the architecture matures and expands.
We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.
If you enrol now you can also get 15% OFF with the promo code “yanprs15”.
Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.
Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!
Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!
Here is a complete list of all my posts on serverless and AWS Lambda. In the meantime, here are a few of my most popular blog posts.
- All you need to know about caching for serverless applications
- Lambda optimization tip – enable HTTP keep-alive
- You are wrong about serverless and vendor lock-in
- You are thinking about serverless costs all wrong
- Just how expensive is the full AWS SDK?
- Check-list for going live with API Gateway and Lambda
- How to choose the right API Gateway auth method
- CloudFormation protip: use !Sub instead of !Join
- AWS Lambda – should you have few monolithic functions or many single-purposed functions?
- Guys, we’re doing pagination wrong
- Top 10 Serverless framework best practices
- How to break the “senior engineer” career ceiling
- My advice to junior developers