[RFC] Allow m_dup() to use JUMBO clusters

[Rick Macklem]

Hans Petter Selasky wrote:
> Hi,
> 
> I'm asking for some input on the attached m_dup() patch, so that
> existing functionality or dependencies are not broken. The background
> for the change is to allow m_dup() to defrag long mbuf chains that
> doesn't fit into a specific hardware's scatter gather entries,
> typically
> when doing TSO.
> 
> In my case the HW limit is 16 entries of length 4K for doing a
> 64KByte
> TSO packet. Currently m_dup() is at best producing 32 entries of each
> 2K
> for a 64Kbytes TSO packet.
> 
> By allowing m_dup() to get JUMBO clusters when allocating mbufs, we
> avoid creating a new function, specific to the hardware, to defrag
> some
> rare-occurring very long mbuf chains into a mbuf chain below 16
> entries.
> 
> Any comments?
> 
1 - If you are using NFS with the default (64K) I/O size, then long
    mbuf chains of 35 entries aren't rare. They happen on every read
    reply/write request.
2 - When I changed NFS to use pagesize clusters for reads/writes I was
    able to get the system into a state where threads were persistently
    stuck on "btalloc". If I understand this correctly, the system was
    not able to allocate boundary tags because the kernel address space
    had been fragmented too much.
    --> As such, I never committed this patch to head and would caution
        against using pagesize clusters.
I do not have a better solution at this point, but I do have an untested
patch (I need to get access to some TSO enabled hardware to test it) that
adds if_hw_tsomaxseg, which is a count of the maximum number of transmit
segments (mbufs in chain) that a network device driver supports.

I think that having the driver set if_hw_tsomaxseg == 16 is preferable to
doing a copy of the data to pagesize clusters. (I'd also say that hardware
that supports only 16 transmit segments for a TSO segment is not a good piece
of hardware for FreeBSD.)

rick

> --HPS
> 
> 
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