linux-kernel - Re: [PATCH] sysctl: reduce ram usage by 40 %

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Message-ID: <m1bpjjewkz.fsf@fess.ebiederm.org>
Date:	Tue, 03 Nov 2009 02:23:08 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>, Greg KH <greg@...ah.com>,
	Benjamin LaHaise <bcrl@...et.ca>,
	Octavian Purdila <opurdila@...acom.com>,
	netdev@...r.kernel.org, Cosmin Ratiu <cratiu@...acom.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysctl: reduce ram usage by 40 %

Eric Dumazet <eric.dumazet@...il.com> writes:

> Eric Dumazet a écrit :
>
>> Its curious because in my tests the biggest problems come from
>> kernel/sysctl.c (__register_sysctl_paths) consuming 80% of cpu
>> in following attempt to create 20.000 devices

I bet that is Al's cute glue all the sysctl data structures together
patch.  It improves readdir and lookup at a small cost at registration
time.

>> (disable hotplug before trying this, and ipv6 too !)
>> modprobe dummy numdummies=20000

>> I believe we should address __register_sysctl_paths() scalability
>> problems too.

Agreed.

>> I dont know what is the 'sentinel' we allocate after each struct ctl_table
>> But I suspect we could reduce size requirement of the 'sentinel' to include
>> only needed fields for the sentinel (and move them at start of ctl_table)

The sentinel is just a NULL terminator.

> Here is the patch to reduce ram usage of sysctl :
>
> [PATCH] sysctl: reduce ram usage by 40 %
>
> We currently reserve space for a so called sentinel, a full struct ctl_table
> for each ctl_table. We can cheat a bit since only needed fields of a sentinel
> are ctl_name and procname. Add a new structure (struct ctl_table_sentinel)
> that includes a full ctl_table and only required part of a sentinel.

Before we address sysctl I would like to get out my patchset that
makes sys_sysctl a wrapper around the ascii version of
/proc/sys/net. Once that goes in it becomes much easier to do things
and perform radical surgery on sysctl.  Little things like .ctl_name and
.strategy go away.

Have you happened to look at the other cost of /proc proper?  Hmm.
Except for /proc/net/dev_snmp6 it doesn't look like we keep per
interface directories in proc so without ivp6 you won't see the proc
generic code at all.

The practical consequence is if /proc/net/dev_snmp6 is not painful during
registration right now we can probably convert all of /proc/sys/net to proc
generic after my other changes are in.

Eric
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