linux-kernel - Re: [PATCH] maple_tree: Avoid checking other gaps after getting the largest gap

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Message-ID: <20231219091454.5ouig5jnvjziol4e@revolver>
Date: Tue, 19 Dec 2023 04:14:54 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Peng Zhang <zhangpeng.00@...edance.com>
Cc: maple-tree@...ts.infradead.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] maple_tree: Avoid checking other gaps after getting the
 largest gap

* Peng Zhang <zhangpeng.00@...edance.com> [231218 21:34]:
> 
> 
> 在 2023/12/19 04:28, Liam R. Howlett 写道:
> > * Liam R. Howlett <Liam.Howlett@...cle.com> [231218 15:20]:
> > > * Peng Zhang <zhangpeng.00@...edance.com> [231215 02:46]:
> > > > The last range stored in maple tree is typically quite large. By
> > > > checking if it exceeds the sum of the remaining ranges in that node, it
> > > > is possible to avoid checking all other gaps.
> > > > 
> > > > Running the maple tree test suite in user mode almost always results in
> > > > a near 100% hit rate for this optimization.
> > > 
> > > This should only be triggered for right-most nodes and root though,
> > > correct (mas->max  == ULONG_MAX from just before this)?
> Yes, only for right-most nodes and root.
> > > 
> > > I wonder if it's worth special case checking the first gap if the node
> > > min is 0 as well.  Might be worth looking at, but this patch is
> > > certainly worth doing.
> > 
> > Actually, not just when the min is 0, we have a special case close to
> > here for slot 0 so we could just check the same sort of thing there.
> I think that the first slot in a node does not have any special
> significance. It has a lower probability of being the largest gap,
> so it may not be worth considering.

It has a higher probability of being a gap, but since there is a special
case for it already it won't be checked unless it is a gap.

The left-most node at each level will probably exhibit the same larger
gap in practice except for the root where it will the be end gap - at
least on x86.

Since these will be hit-cache I'm not sure either will make much of a
practical difference though.

> > 
> > > 
> > > > 
> > > > Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
> > > 
> > > Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > > 
> > > > ---
> > > >   lib/maple_tree.c | 3 +++
> > > >   1 file changed, 3 insertions(+)
> > > > 
> > > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > > > index c9a970ea20dd..6f241bb38799 100644
> > > > --- a/lib/maple_tree.c
> > > > +++ b/lib/maple_tree.c
> > > > @@ -1518,6 +1518,9 @@ static unsigned long mas_leaf_max_gap(struct ma_state *mas)
> > > >   		gap = ULONG_MAX - pivots[max_piv];
> > > >   		if (gap > max_gap)
> > > >   			max_gap = gap;
> > > > +
> > > > +		if (max_gap > pivots[max_piv] - mas->min)
> > > > +			return max_gap;
> > > >   	}
> > > >   	for (; i <= max_piv; i++) {
> > > > -- 
> > > > 2.20.1
> > > > 
> >