linux-kernel - Re: [PATCH v2] zswap: replace RB tree with xarray

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Date: Tue, 5 Mar 2024 03:08:15 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: David Laight <David.Laight@...lab.com>
Cc: "'Chris Li'" <chrisl@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	Nhat Pham <nphamcs@...il.com>, Johannes Weiner <hannes@...xchg.org>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Chengming Zhou <zhouchengming@...edance.com>, 
	Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v2] zswap: replace RB tree with xarray

On Fri, Mar 01, 2024 at 09:58:43PM +0000, David Laight wrote:
> From: Chris Li
> > Sent: 29 February 2024 08:46
> > 
> > Very deep RB tree requires rebalance at times. That
> > contributes to the zswap fault latencies. Xarray does not
> > need to perform tree rebalance. Replacing RB tree to xarray
> > can have some small performance gain.
> > 
> > One small difference is that xarray insert might fail with
> > ENOMEM, while RB tree insert does not allocate additional
> > memory.
> 
> What is the difference in kernel memory use?
> IIRC someone pointed out (in the rosebush thread) that xarray
> uses a lot of kernel memory if the items are randomly distributed.

I often thought it would be useful if there is an API for the xarray to
report how much memory it's using. For zswap, we could plumb this to a
debugfs interface or so to measure the tree usage and compare to the
rbtree. The cost is fixed with the rbtree IIUC, 3 words per zswap entry.