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Message-ID: <55B66C1A.4000804@redhat.com>
Date: Mon, 27 Jul 2015 10:36:26 -0700
From: Alexander Duyck <alexander.h.duyck@...hat.com>
To: David Ahern <dsa@...ulusnetworks.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: increase in time to delete an interface with 4.x kernels
On 07/27/2015 09:49 AM, David Ahern wrote:
> Hi Alex:
>
> I believe you did the recent overhaul to the fib implementation. I am
> seeing dramatically higher times to delete an interface with an ipv4
> address in 4.2-rc3. perf-top points to update_suffix:
>
> PerfTop: 15834 irqs/sec kernel:97.3% exact: 0.0% [4000Hz
> cpu-clock], (all, 4 CPUs)
> -------------------------------------------------------------------------------------------
>
>
> 74.69% [kernel] [k] update_suffix
> 2.38% [kernel] [k] fib_table_flush
> 2.20% [kernel] [k] fib6_walk_continue
> 2.03% [kernel] [k] fib6_ifdown
> 1.31% [kernel] [k] fib6_age
>
>
> I have a simple script to create and assign an ipv4 address to 10k dummy
> interfaces:
>
> l=0
> for (( j = 1; j <= 40; j += 1))
> do
> for (( k = 1 ; k <= 250 ; k += 1 ))
> do
> l=$((l + 1))
> ip link add dev dummy${l} type dummy
> ip addr add 72.$j.$k.1/24 dev dummy${l}
> ifconfig dummy${l} up
> done
> done
>
>
> and a counter script to delete them all:
>
> k=$(ip link show | grep dummy | wc -l)
> for (( j = 1; j <= k; j += 1))
> do
> ip link del dev dummy${j}
> done
>
Okay so looking over what this script does it looks like it really
exposes the worst case scenerio for update_suffix. You have a monstrous
tnode that is 15 bits ins size. That is roughly 32K entries, and
unfortunately the suffix is 8 bits long with a position of 7.
The result is that for every removal the code is scanning 16K entries in
order to relevel things after an entry is removed.
Let me try a couple of quick things and I should have a patch for you in
the next couple of hours.
Thanks.
- Alex
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