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

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
        for (i = 0; i<  count; i++) {
                if (tx_pool)
                        vif = netbk->gnttab_tx_vif[i];
                        vif = netbk->gnttab_rx_vif[i];

                pers_entry = vif->persistent_gnt;
                gnt_count =&vif->persistent_gntcnt;
                gnt_total = MAXIMUM_OUTSTANDING_BLOCK_REQS;
        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.



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