Re: [Xen-devel] [PATCH 2/4] xen/netback: Split one page pool into two(tx/rx) page pool.

[ANNIE LI <annie.li@xxxxxxxxxx>]

On 2012-11-15 17:15, Ian Campbell wrote:
> On Thu, 2012-11-15 at 07:04 +0000, Annie Li wrote:
> > For tx path, this implementation simplifies the work of searching out
> > grant page from page pool based on grant reference.
> It's still a linear search though, and it doesn't look much simpler to
> me:
>         for (i = 0; i<  count; i++) {
>                 if (tx_pool)
>                         vif = netbk->gnttab_tx_vif[i];
>                 else
>                         vif = netbk->gnttab_rx_vif[i];
>
>                 pers_entry = vif->persistent_gnt;
>                 gnt_count =&vif->persistent_gntcnt;
>                 gnt_total = MAXIMUM_OUTSTANDING_BLOCK_REQS;
> becomes:
>         for (i = 0; i<  count; i++) {
>                 if (tx_pool) {
>                         vif = netbk->gnttab_tx_vif[i];
>                         gnt_count =&vif->persistent_tx_gntcnt;
>                         gnt_total = XEN_NETIF_TX_RING_SIZE;
>                         pers_entry = vif->persistent_tx_gnt;
>                 } else {
>                         vif = netbk->gnttab_rx_vif[i];
>                         gnt_count =&vif->persistent_rx_gntcnt;
>                         gnt_total = 2*XEN_NETIF_RX_RING_SIZE;
>                         pers_entry = vif->persistent_rx_gnt;
>                 }
Yes, the code is not simpler. If we make netback per-VIF based, then 
these code will disappear.
The simplifying here means for tx path, the max search index is 
XEN_NETIF_TX_RING_SIZE(256 here), and this change can save some time 
when searching out grant page for specific grant reference.
> > @@ -111,8 +109,16 @@ struct xenvif {
> >
> >         wait_queue_head_t waiting_to_free;
> >
> > -       struct persistent_entry *persistent_gnt[MAXIMUM_OUTSTANDING_BLOCK_REQS];
> > -       unsigned int persistent_gntcnt;
> > +       struct persistent_entry *persistent_tx_gnt[XEN_NETIF_TX_RING_SIZE];
> > +
> > +       /*
> > +        * 2*XEN_NETIF_RX_RING_SIZE is for the case of each head/fragment page
> Shouldn't that been incorporated into MAXIMUM_OUTSTANDING_BLOCK_REQS
> (sic) too?
Yes, the total value is same as MAXIMUM_OUTSTANDING_BLOCK_REQS. But here

2*XEN_NETIF_RX_RING_SIZE means it is only used by rx path, and it is used just 
like other elements in netback structure, such as grant_copy_op, meta, etc.


> > +        * using 2 copy operations.
> > +        */
> > +       struct persistent_entry *persistent_rx_gnt[2*XEN_NETIF_RX_RING_SIZE];
> What is the per-vif memory overhead after this change?
Per-vif memory overhead is following,
for tx path, it is about XEN_NETIF_RX_RING_SIZE*PAGE_SIZE  (256 
PAGE_SIZE here)
for rx path, it is about 2*XEN_NETIF_RX_RING_SIZE*PAGE_SIZE  (512 
PAGE_SIZE here)
I can add some comment here.

Thanks
Annie
> Ian.
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