[Devel] Re: Performance of netns with sysfs

[Guenter Roeck]

There are two key elements affecting performance when creating large
numbers of interfaces:
- full_name_hash() doesn't do a good job in selecting hash buckets
- sysfs uses a linked list to store directory entries, and strcmp() to
compare entries.

The sysfs problem may be solved with Eric's sysfs upcoming changes; if
not, there are (at least) two ways to improve sysfs performance: 
1) store a name hash with each entry, and compare the hash result before
comparing the entire name
2) create a per-directory hash table.

full_name_hash() can be improved by replacing the hash, for example as
follows. 
Old:
	return (prevhash + (c << 4) + (c >> 4)) * 11;
New:
	return (prevhash + c) * 41;

Guenter

On Thu, 2009-01-15 at 12:57 -0800, Dan Smith wrote:
> There has been some discussion lately about network namespaces and the
> interaction with sysfs.  With the introduction of Guenter's ipcgroup
> patches, there was also the question of how feasible it is to create
> thousands of network namespaces.  So, I decided to see if I could even
> create thousands of veth pairs, and if so move them into thousands of
> network namespaces.  I was pleased to see that the system didn't fall
> over, but found that the process slowed significantly with higher
> numbers if sysfs was enabled.  I thought it would be prudent to post
> some numbers.
> 
> I first tested creating 1000 and 2500 veth pairs, attaching one side
> to a bridge with and without sysfs.  Next I created 2500 network
> namespaces, along with 2500 veth pairs.  One side of each pair was
> attached to a bridge and the other was moved into the namespace.  The
> results are:
> 
>   1000 veth pairs:        8x slower with CONFIG_SYSFS
>   2500 veth pairs         4.5x slower
>   2500 netns, veth pairs: 6x slower
> 
> The tests were done with hal disabled, attaching every third veth
> device to a different bridge (to overcome the limit of 1023 taps per
> bridge).
> 
> --
> Dan Smith
> IBM Linux Technology Center
> email: danms at us.ibm.com
> 

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