Zack's Kernel News

Zack's Kernel News

Article from Issue 210/2018
Author(s):

Zack Brown reports on improving a hashing function, constant values adjustable at boot time, and dealing with an Intel design flaw. 

Improving a Hashing Function

Amir Goldstein posted a small fix to a kernel function that would produce a unique hash value for an input string. He noticed that for 64-bit values, some bits were lost before calculating the hash – resulting in a slightly less good hash. For 32-bit values, he said, no bits were lost, and the code was left unchanged, but since the relevant stringhash.h file had no official maintainer, and Linus Torvalds had done some work on it in the past, Amir asked Linus what he thought of the fix and for advice on how to test it. As Amir put it, "I wouldn't even know where to begin testing its affects, or how to prove if there really is a problem."

Linus dug back into his memory, saying he believed the lost bits were intentional, though he acknowledged, "it's a long time ago, and we've changed some of the hashing since."

But Linus also said, "you're wrong that it's a no-op on 32-bit. It's a very expensive and pointless multiplication there too, even if the shift ends up being a no-op. The name hashing is pretty performance-sensitive."

[...]

Use Express-Checkout link below to read the full article (PDF).

Buy this article as PDF

Express-Checkout as PDF
Price $2.95
(incl. VAT)

Buy Linux Magazine

SINGLE ISSUES
 
SUBSCRIPTIONS
 
TABLET & SMARTPHONE APPS
Get it on Google Play

US / Canada

Get it on Google Play

UK / Australia

Related content

comments powered by Disqus
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters

Support Our Work

Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.

Learn More

News