...
Homa’s matching mechanism does come at a price: it consumes TOR buffer space. There are two elements of Homa that result in buffer occupancy. The first is unscheduled packets: if several senders start sending messages to the same receiver at the same time, buffers will accumulate in the TOR. The receiver will drain those buffers as quickly as possible by withholding grants, but there is no upper limit on how many buffers could accumulate in a pathological case.
The second cause of buffer occupancy is overcommitment, where Homa intentionally arranges for buffers to accumulate in the TOR; this is necessary to ensure high utilization of the downlink, if the current sender should stop transmitting.
Fortunately, current switches have enough buffer space to meet Homa’s needs in practice. See this page for more detailsFor a full discussion of Homa’s usage of buffers (and whether it is problematic), see this page.
[1] J. Perry, A. Ousterhout, H. Balakrishnan, D. Shah, and H. Fugal, “Fastpass: A Centralized “Zero-Queue” Datacenter Network”, SIGCOMM 2014, August 17-22, 2014, Chicago, IL, pp. 307-318.