On Wed, 31 Oct 2018 00:24:36 -0700 Florin Coras <fcoras.li...@gmail.com> wrote:
> >>>> No reader-writer locks are 100's of times slower. In fact reader > >>>> write locks are slower than normal spin lock. > >>>> > >>> > >>> I guess you meant that in general, and I can see how for scenarios with > >> multiple writers and readers performance can be bad. But from your original > >> message I assumed you’re mostly interested in concurrent read performance > >> with few writes. For such scenarios I would expect our current, simple, > >> spin > >> and rw lock implementations to not be that bad. If that’s provably not the > >> case, > >> we should definitely consider doing RCU. > >>> > >>> Also worth noting that a common pattern in vpp is to have per thread data > >>> > >> structures and then entirely avoid locking. For lookups we typically use > >> the > >> bihash and that is thread safe. > > When you say 'per thread data structures', does it mean the data structures > > will be duplicated for each data plane thread? > > No, we don’t duplicate the data. Instead, we rely on RSS hashing to pin flows > to workers and then build per worker state. > > For scenarios when that doesn’t work, we handoff flows between workers. Ok, the tradeoff is that having a single worker is a bottleneck, and if packet arrives on one core and then is processed on another core there is a cache miss. Per the original discussion, a reader/writer lock even uncontended requires an atomic increment, and that increment is a locked instruction and a cache miss with multiple readers. RCU has zero overhead for readers. The problem is pushed to the writer to deal with. It works fine for data structures like lists or trees that can be updated with a specific access pattern such that reader always sees valid data. The case I was thinking of is things like flow and routing tables.
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