Re: [PATCH v2 01/16] iscsi-target: add callback to alloc and free PDU

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On Wed, Apr 13, 2016 at 12:56:25PM +0300, Sagi Grimberg wrote:
> 
> >>On 09/04/16 16:11, Varun Prakash wrote:
> >>>Add two callbacks to struct iscsit_transport -
> >>>
> >>>1. void *(*iscsit_alloc_pdu)()
> >>>    iscsi-target uses this callback for
> >>>    iSCSI PDU allocation.
> >>>
> >>>2. void (*iscsit_free_pdu)
> >>>    iscsi-target uses this callback
> >>>    to free an iSCSI PDU which was
> >>>    allocated by iscsit_alloc_pdu().
> >>
> >>Can you please explain why are you adding two different
> >>callouts? Who (Chelsio T5) will need it, and why they can't
> >>use the in-cmd pdu?
> >
> >I am adding these to avoid per PDU 48 bytes(BHS) memcpy from cmd->pdu to
> >transport driver tx buffer, iscsi-target can directly form iscsi hdr
> >in transport driver tx buffer.
> 
> Is that what it's for? Looks meaningless to me, do
> you have an indication that this is some sort of bottleneck?
> 
> I see you memset in your pdu_alloc, where is the gain
> here anyway?

We can definitely avoid memcpy if iscsi-target fills all
the fields in iscsi hdr, in current code some fields
are assumed to be zero so a memset is required in transport
driver.
I will drop this patch in -v3, thanks. 
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