On Thu, Feb 14, 2019 at 10:16:52PM +0000, Wei Yang wrote: > On Wed, Feb 13, 2019 at 08:12:58AM -0800, Matthew Wilcox wrote: > >The only remaining user of the radix tree in that tree is the IDR. So > >now I'm converting the IDR users over to the XArray as well. > > Wow, really a HUGE work. Yes ... but necessary. Have to pay down the technical debt. > >But that isn't what I was talking about. At the moment, the radix > >tree and the XArray use the same data structure. It has good best-case > >performance, but shockingly bad worst-case performance. So we're looking > >at replacing the data structure, which won't require changing any of the > >users (maybe the page cache ... that has some pretty intimate knowledge > >of exactly how the radix tree works). > > Two questions from my curiosity: > > 1. Why you come up the idea to replace radix tree with XArray even they > use the same data structure? The radix tree API was just awful to use. I tried to convert some users with their own resizing-array-of-pointers to use the radix tree, and I gave up in disgust. I believe the XArray is much simpler to use. > 2. The worst-case performance is related to the data structure itself? Yes. Consider the case where you store a pointer at its own address in the data structure. It'll allocate 11 nodes occupying one-and-a-half pages of RAM in order to store a single pointer. > >> BTW, have we compared the performance difference? > > > >It's in the noise. Sometimes the XArray does a little better because > >the APIs encourage the user to do things in a more efficient way. > >Some of the users are improved just because the original author didn't > >know about a more efficient way of doing what they wanted to do. > > So sometimes XArray does a little worse? > > Why this happens whey XArray and radix tree has the same data structure? > > Interesting. I'm not sure there are any cases where the XArray does worse. Feel free to run your own measurements ... the test cases in tools/testing/radix-tree can always do with being improved ;-) (that directory is a bit misnamed as it now features tests for the radix tree, IDR, IDA and XArray).
