Re: [PATCH] xarray: Document erasing entries during iteration

[Matthew Wilcox]

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).