On 12/12/17 20:07, Stephen Hemminger wrote: > On Tue, 12 Dec 2017 16:02:50 +0200 > Nikolay Aleksandrov <niko...@cumulusnetworks.com> wrote: > >> Before this patch the bridge used a fixed 256 element hash table which >> was fine for small use cases (in my tests it starts to degrade >> above 1000 entries), but it wasn't enough for medium or large >> scale deployments. Modern setups have thousands of participants in a >> single bridge, even only enabling vlans and adding a few thousand vlan >> entries will cause a few thousand fdbs to be automatically inserted per >> participating port. So we need to scale the fdb table considerably to >> cope with modern workloads, and this patch converts it to use a >> rhashtable for its operations thus improving the bridge scalability. >> Tests show the following results (10 runs each), at up to 1000 entries >> rhashtable is ~3% slower, at 2000 rhashtable is 30% faster, at 3000 it >> is 2 times faster and at 30000 it is 50 times faster. >> Obviously this happens because of the properties of the two constructs >> and is expected, rhashtable keeps pretty much a constant time even with >> 10000000 entries (tested), while the fixed hash table struggles >> considerably even above 10000. >> As a side effect this also reduces the net_bridge struct size from 3248 >> bytes to 1344 bytes. Also note that the key struct is 8 bytes. >> >> Signed-off-by: Nikolay Aleksandrov <niko...@cumulusnetworks.com> >> --- > > Thanks for doing this, it was on my list of things that never get done. > > Some downsides: > * size of the FDB entry gets larger.
It does not, due to smp alignment of the write-heavy members we had a large hole between cache line 1 and 2, the new 8 bytes fit perfectly and there are still bytes left to use. > * you lost the ability to salt the hash (and rekey) which is important > for DDoS attacks The hash is always salted (property of rhashtable) and in fact is better because now the salt is generated for each rhashtable separately rather than having 1 global salt for all bridge devices. > * being slower for small (<10 entries) also matters and is is a common > use case for containers. I think they're pretty comparable in speed, the difference is negligible IMO.
