On 2021-11-19 10:07, Saku Ytti via juniper-nsp wrote: > Cut-through does nothing, because your egress is congested, you can > only use cut-through if egress is not congested.
Cut-through actually *can* help a little bit. The buffer space in the Trident and Tomahawk chips is mostly shared between all ports; only a small portion of it is dedicated per port[1]. If you have lots of traffic on some ports, with little or no congestion, enabling cut-through will leave more buffer space available for the congested ports, as the packets will leave the switch/router quicker. One should note though that these chips will fall back to store- and-forward if the ingress port and egress port run at different speeds. (In theory, it should be possible to do cut-through as long as the egress port is not faster than the ingress port, but as far as I know, any speed mismatch causes store-and-forward to be used). Also, if you have rate limiting or shaping enabled on the ingress or egress port, the chips will fall back to store-and-forward. Whether this helps *enough*, is another question. :-) I believe in general, it will only make a pretty small difference in buffer usage. I enabled cut-through forwarding on our QFX5xxx:es and EX4600:s a few years ago, and any change in packet drop rates or TCP performance (both local and long-distance) was lost way down in the noise. But I have seen reports from others that saw a meaningful, if not exactly huge, difference; that was several years ago, though, and I didn't save any reference to the report, so you might want to classify that as hearsay... (I have kept cut-through enabled on our devices, since I don't know of any practical disadvantages, and it *might* help a tiny little bit in some cases.) [1] Of the 12 Mbyte buffer space in Trident 2, which is used in QFX5100 and EX4600, 3 Mbyte is used for per-port dedicated buffers, and 9 Mbyte is shared between all ports. I believe on later chips an even larger percentage is shared. -- Thomas Bellman, National Supercomputer Centre, Linköping Univ., Sweden "We don't understand the software, and sometimes we don't understand the hardware, but we can *see* the blinking lights!"
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