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

From: Eric Dumazet
Date: Tue Nov 03 2009 - 01:14:55 EST

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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