Skip to main content

Computer Science: Arbitrarily Fine Locking

This is a relatively simple idea concerning mutex usage. I imagine someone else has probably thought of it before. However, since I just thought of it, I figured I'd blog it. I have no clue why I was thinking about mutexes. I usually prefer share-nothing approaches like Erlang. Note, I am specifically not trying to comment on Python's GIL.

Imagine you have a bunch of resources (like objects or structs) that you wish to protect. One way to protect them is to use a single mutex. This is called coarse-grained locking. At the opposite end of the spectrum, you can create a new mutex for every resource. This is called fine-grained locking. However, what if you want something in the middle?

Having a single lock is unfortunate because it forces a lot of your code to be effectively single-threaded, even if you have multiple processors. However, perhaps creating a new mutex for every single resource might be overkill. (Personally, I'm unsure of why that might be the case.)

Here's an approach to get arbitrarily fine locking. Create N mutexes (where N is tunable). Protect each resource using mutex number resource_id % N. The resource_id could be whatever, as long as it's unique. Perhaps it's the index of an array, or perhaps it's a pointer to the resource in memory.

And now for something completely different! The best part of Lisp is that it has garbage collection. It recycles garbage so that you can grow new trees ;)

Comments

Unknown said…
Your assigning a resource to a lock by hashing in the hopes that you'll have a large pool of resources more or less randomly distributed across a smaller pool of locks. Clever, but I'm still not sure what problem this solves.

Don't forget: you've got to assure consistent ordering of lock acquisition. Otherwise you're setting yourself up for deadlocks if you have tasks that require more than one resource at a time.

Also: acquiring and releasing locks is not free. This will place an upper bound on how many you'd want to use in your system.
jjinux said…
> Clever, but I'm still not sure what problem this solves.

I'm not either. I'm not sure why it even came to mind.

> Don't forget: you've got to assure consistent ordering of lock acquisition. Otherwise you're setting yourself up for deadlocks if you have tasks that require more than one resource at a time.

Yep.

> Also: acquiring and releasing locks is not free. This will place an upper bound on how many you'd want to use in your system.

Yep.
Bill Mill said…
> The resource_id could be whatever, as long as it's unique

It needs to be unique and evenly distributed with respect to mod N, not just unique.
jjinux said…
> It needs to be unique and evenly distributed with respect to mod N, not just unique.

Agreed, although you can pull the same tricks that you pull with hashes. If you get too many collisions on any one mutex, you can increase N.
Anonymous said…
It is not a bad idea, but it is not new either. We have been doing this a lot when we implement hash tables for dynamic hashing (linked lists growing out of buckets). You have a lock per bucket, so that insert and delete operations can work atomically. But if you have many buckets and many cores/threads, many of those operations can take place at the same time, just not on the same bucket.

So, yeah, it works and has the desired effect, but it has been done many times before.
Anonymous said…
This is used in the FreeBSD kernel in some places where a mutex is needed but either allocating one statically or at runtime would have too much time or space overhead.

We keep a "pool" of mutexes that can be used by anyone and are hashed based on a resource address. To get around the deadlock issue pool mutexes must be leaf mutexes, i.e. you are not allowed to acquire other locks while holding them.
jjinux said…
> It is not a bad idea, but it is not new either.

> This is used in the FreeBSD kernel in some places

Excellent comments. Thanks!

Ideas are rarely new, so I'm always glad to hear when I had a good, existing idea rather than a bad, existing idea ;)

Popular posts from this blog

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python . Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening. The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for B

Ubuntu 20.04 on a 2015 15" MacBook Pro

I decided to give Ubuntu 20.04 a try on my 2015 15" MacBook Pro. I didn't actually install it; I just live booted from a USB thumb drive which was enough to try out everything I wanted. In summary, it's not perfect, and issues with my camera would prevent me from switching, but given the right hardware, I think it's a really viable option. The first thing I wanted to try was what would happen if I plugged in a non-HiDPI screen given that my laptop has a HiDPI screen. Without sub-pixel scaling, whatever scale rate I picked for one screen would apply to the other. However, once I turned on sub-pixel scaling, I was able to pick different scale rates for the internal and external displays. That looked ok. I tried plugging in and unplugging multiple times, and it didn't crash. I doubt it'd work with my Thunderbolt display at work, but it worked fine for my HDMI displays at home. I even plugged it into my TV, and it stuck to the 100% scaling I picked for the othe

Creating Windows 10 Boot Media for a Lenovo Thinkpad T410 Using Only a Mac and a Linux Machine

TL;DR: Giovanni and I struggled trying to get Windows 10 installed on the Lenovo Thinkpad T410. We struggled a lot trying to create the installation media because we only had a Mac and a Linux machine to work with. Everytime we tried to boot the USB thumb drive, it just showed us a blinking cursor. At the end, we finally realized that Windows 10 wasn't supported on this laptop :-/ I've heard that it took Thomas Edison 100 tries to figure out the right material to use as a lightbulb filament. Well, I'm no Thomas Edison, but I thought it might be noteworthy to document our attempts at getting it to boot off a USB thumb drive: Download the ISO. Attempt 1: Use Etcher. Etcher says it doesn't work for Windows. Attempt 2: Use Boot Camp Assistant. It doesn't have that feature anymore. Attempt 3: Use Disk Utility on a Mac. Erase a USB thumb drive: Format: ExFAT Scheme: GUID Partition Map Mount the ISO. Copy everything from