linux-kernel - Re: [PATCH] Add next, prev pointer in xa

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Date:   Tue, 17 Mar 2020 06:40:43 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     JaeJoon Jung <rgbi3307@...il.com>
Cc:     maple-tree@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-riscv <linux-riscv@...ts.infradead.org>, koct9i@...il.com
Subject: Re: [PATCH] Add next, prev pointer in xa_node at the lib/xarray.c

On Tue, Mar 17, 2020 at 04:32:34PM +0900, JaeJoon Jung wrote:
> Hi Matthew,
> I add next, prev pointer in the xa_node to improve search performance.
> XArray currently has the following search capabilities:
> 
> Search algorithm performance is O(n) = n x log(n)

That's not how "big-O" notation is used.
https://en.wikipedia.org/wiki/Big_O_notation

What you mean to say here is O(n.log(n)).

> For example,
> XA_CHUNK_SHIFT 2
> XA_CHUNK_SIZE 4

I'm not really interested in the performance of a cut-down radix tree.
You can re-run the numbers with a SHIFT of 6 and SIZE of 64 to get a
more useful set of performance numbers.

> If you connect the leaf node with the next and prev pointers as follows,
> the search performance is improved to O (n) = n.

But that's not free.  Right now, the xa_node is finely tuned in
size to 576 bytes on 64-bit systems (32-bit systems are a secondary
consideration).  That gets us 7 nodes per page.  Increasing the size any
further reduces the number per page to 6, which is a pretty meaningful
increase in memory usage.

> @@ -1125,6 +1125,8 @@ struct xa_node {
>         unsigned char   count;          /* Total entry count */
>         unsigned char   nr_values;      /* Value entry count */
>         struct xa_node __rcu *parent;   /* NULL at top of tree */
> +        struct xa_node __rcu *prev;     /* previous node pointer */
> +        struct xa_node __rcu *next;     /* next node pointer */

You should be indenting with tabs, not spaces.  Or your mail system is
messing with your patches.