On 8/17/17, 5:22 AM, "Jan Scheurich" <[email protected]> wrote:
The RSS hash threshold method looks like the only pseudo-random
criterion that we can use that produces consistent result for every packet of a
flow and does require more information. Of course elephant flows with an
unlucky hash value might never get to use the EMC, but that risk we have with
any stateless selection scheme.
[Darrell] It is probably something I know by another name, but JTBC, can
you define the “RSS hash threshold method” ?
I am referring to Billy's proposal
(https://urldefense.proofpoint.com/v2/url?u=https-3A__mail.openvswitch.org_pipermail_ovs-2Ddev_2017-2DAugust_336509.html&d=DwIGaQ&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=dGZmbKhBG9tJHY4odedsGA&m=_i_IqWudJqU7R3_ZaFm7HHhOQMHwm_U6G-EIyOGjkxI&s=IDMHVf9n5CjmHMI67mzMd0HZegNJ_LntZLfcdpRUvJI&e=
)
In essence the is suggests to only select packets for EMC lookup whose RSS
hash is above a certain threshold. The lookup probability is determined by the
threshold (e.g. threshold of 0.75 * UINT32_MAX corresponds to 25%). It is
pseudo-random as, assuming that the hash result is uniformly distributed, flows
will profit from EMC lookup with the same probability.
[Darrell] ahh, there is no actual patch yet, just an e-mail
I see, you have a coined the term “RSS hash threshold method”
for the approach; the nomenclature makes sense now.
I’ll have separate comments, of course, on the proposal itself.
The new thing required will be the dynamic adjustment of lookup
probability to the EMC fill level and/or hit ratio.
[Darrell] Did you mean insertion probability rather than lookup probability
?
No, I actually meant dynamic adaptation of lookup probability. We don't
want to reduce the EMC lookup probability when the EMC is not yet overloaded,
but only when the EMC hit rate degrades due to collisions. When we devise an
algorithm to adapt lookup probability, we can study if it could make sense to
also dynamically adjust the currently fixed (configurable) EMC insertion
probability based on EMC fill level and/or hit rate.
[Darrell] Now that I know what you are referring to above, it is a lot easier
to make the linkage.
BR, Jan
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