I'm confused. t_maxsegsize is basically the connection's MSS, eg, what the NIC is supposed to break the TSO into. However if_hw_tsomaxsegsize is the max size of the TSO to be broken up. Eg, on a NIC without oddball limitations, if_hw_tsomaxsegcount might be 64, if_hw_tsomaxsegsize might be 65535. That means that TSO is supposed to send a max of 64K down to the NIC, in a max of 64 packets. If t_maxseg is less than 1000, then we'd have to limit the TSO to 64 packets.

It seems like by restricting if_hw_tsomaxsegsize to t_maxseg, you're essentially turning off TSO, since you're only allowing one MSS to be sent at a time.

In D6656#578326, @gallatin wrote:

I'm confused. t_maxsegsize is basically the connection's MSS, eg, what the NIC is supposed to break the TSO into. However if_hw_tsomaxsegsize is the max size of the TSO to be broken up. Eg, on a NIC without oddball limitations, if_hw_tsomaxsegcount might be 64, if_hw_tsomaxsegsize might be 65535. That means that TSO is supposed to send a max of 64K down to the NIC, in a max of 64 packets. If t_maxseg is less than 1000, then we'd have to limit the TSO to 64 packets.

It seems like by restricting if_hw_tsomaxsegsize to t_maxseg, you're essentially turning off TSO, since you're only allowing one MSS to be sent at a time.

Ugh, I'm wrong. I was confusing t_tsomax with t_tsomaxsegsize. I forgot that some NICs are apparently so broken that they limit TSO seg sizes.

In D6656#578863, @gallatin wrote:

In D6656#578326, @gallatin wrote:

I'm confused. t_maxsegsize is basically the connection's MSS, eg, what the NIC is supposed to break the TSO into. However if_hw_tsomaxsegsize is the max size of the TSO to be broken up. Eg, on a NIC without oddball limitations, if_hw_tsomaxsegcount might be 64, if_hw_tsomaxsegsize might be 65535. That means that TSO is supposed to send a max of 64K down to the NIC, in a max of 64 packets. If t_maxseg is less than 1000, then we'd have to limit the TSO to 64 packets.

It seems like by restricting if_hw_tsomaxsegsize to t_maxseg, you're essentially turning off TSO, since you're only allowing one MSS to be sent at a time.

Ugh, I'm wrong. I was confusing t_tsomax with t_tsomaxsegsize. I forgot that some NICs are apparently so broken that they limit TSO seg sizes.

So this is correct? Or am I missing something else?

In D6656#579330, @royger wrote:

In D6656#578863, @gallatin wrote:

In D6656#578326, @gallatin wrote:

I'm confused. t_maxsegsize is basically the connection's MSS, eg, what the NIC is supposed to break the TSO into. However if_hw_tsomaxsegsize is the max size of the TSO to be broken up. Eg, on a NIC without oddball limitations, if_hw_tsomaxsegcount might be 64, if_hw_tsomaxsegsize might be 65535. That means that TSO is supposed to send a max of 64K down to the NIC, in a max of 64 packets. If t_maxseg is less than 1000, then we'd have to limit the TSO to 64 packets.

It seems like by restricting if_hw_tsomaxsegsize to t_maxseg, you're essentially turning off TSO, since you're only allowing one MSS to be sent at a time.

Ugh, I'm wrong. I was confusing t_tsomax with t_tsomaxsegsize. I forgot that some NICs are apparently so broken that they limit TSO seg sizes.

So this is correct? Or am I missing something else?

I think it is correct, but I would prefer somebody from the transport team to review it.

Hi Drew,

t_tsomaxsegsize usually only affects the number of DMA segments and the mbuf chain. At least the Mellanox NICS do it this way.

How does t_maxseg affect the generated DMA chain?

--HPS

I mean DMA -> mbuf chain.