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

From: Eric W. Biederman
Date: Tue Nov 03 2009 - 17:28:48 EST

Greg KH <greg@xxxxxxxxx> writes:

> On Tue, Nov 03, 2009 at 07:14:33AM +0100, Eric Dumazet wrote:
>> Greg KH a ?crit :
>> > 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.
> But registering 20000 devices is a far different problem from using
> those 20000 devices :)
> I think the "use the device" path should be the one we care the most
> about fixing up, as that is much more common than the register path for
> all users.

Definitely. Of the three proc sysctl and sysfs. sysctl tends to have
the worst costs across the board. They are all rarely used so a lot
of what gets hit when scaling are rare path events that even the most
horrible code works fine on small systems.

Usually slow registration times indicate an O(N^2) or worse data
structure for filename lookup.

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