netdev - Re: [PATCH 1/3] sysfs directory scaling: rbtree for dirent name lookups

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Date:	Tue, 03 Nov 2009 07:14:33 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Greg KH <greg@...ah.com>
CC:	Benjamin LaHaise <bcrl@...et.ca>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Octavian Purdila <opurdila@...acom.com>,
	netdev@...r.kernel.org, Cosmin Ratiu <cratiu@...acom.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sysfs directory scaling: rbtree for dirent name lookups

Greg KH a écrit :
> On Sun, Nov 01, 2009 at 11:31:30AM -0500, Benjamin LaHaise wrote:
>> Use an rbtree in sysfs_dirent to speed up file lookup times
>>
>> Systems with large numbers (tens of thousands and more) of network 
>> interfaces stress the sysfs code in ways that make the linear search for 
>> a name match take far too long.  Avoid this by using an rbtree.
> 
> What kind of speedups are you seeing here?  And do these changes cause a
> memory increase due to the structure changes which outweigh the
> speedups?
> 
> What kind of test are you doing to reproduce this?
> 

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

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

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

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)

        /*
         * For each path component, allocate a 2-element ctl_table array.
         * The first array element will be filled with the sysctl entry
         * for this, the second will be the sentinel (ctl_name == 0).
         *
         * We allocate everything in one go so that we don't have to
         * worry about freeing additional memory in unregister_sysctl_table.
         */
        header = kzalloc(sizeof(struct ctl_table_header) +
                         (2 * npath * sizeof(struct ctl_table)), GFP_KERNEL);

Then, adding an rb_node in ctl_table_header to speedup __register_sysctl_paths() a bit
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