Dear Russell
On Thu, Apr 3, 2014 at 11:27 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Wed, Apr 02, 2014 at 11:21:45AM +0200, Arnd Bergmann wrote:
>> - As David Laight pointed out earlier, you must also ensure that
>> you don't have too much /data/ pending in the descriptor ring
>> when you stop the queue. For a 10mbit connection, you have already
>> tested (as we discussed on IRC) that 64 descriptors with 1500 byte
>> frames gives you a 68ms round-trip ping time, which is too much.
>> Conversely, on 1gbit, having only 64 descriptors actually seems
>> a little low, and you may be able to get better throughput if
>> you extend the ring to e.g. 512 descriptors.
>
> You don't manage that by stopping the queue - there's separate interfaces
> where you report how many bytes you've queued (netdev_sent_queue()) and
> how many bytes/packets you've sent (netdev_tx_completed_queue()). This
> allows the netdev schedulers to limit how much data is held in the queue,
> preserving interactivity while allowing the advantages of larger rings.
My god, it's awesome.
The latency can be solved via adding netdev_sent_queue in xmit, and
netdev_completed_queue in reclaim.
In the experiment,
iperf -P 3 could get 930M, and ping could get response within 0.4 ms
in the meantime.
Is that mean the timer -> reclaim should be removed at all?
The background are.
1. No xmit_complete interrupt.
2. Only xmit call reclaim used buffer can achieve best throughput.
Adding timer in case no next xmit for reclaim.
>
>> > + phys = dma_map_single(&ndev->dev, skb->data, skb->len, DMA_TO_DEVICE);
>> > + if (dma_mapping_error(&ndev->dev, phys)) {
>> > + dev_kfree_skb(skb);
>> > + return NETDEV_TX_OK;
>> > + }
>> > +
>> > + priv->tx_skb[tx_head] = skb;
>> > + priv->tx_phys[tx_head] = phys;
>> > + desc->send_addr = cpu_to_be32(phys);
>> > + desc->send_size = cpu_to_be16(skb->len);
>> > + desc->cfg = cpu_to_be32(DESC_DEF_CFG);
>> > + phys = priv->tx_desc_dma + tx_head * sizeof(struct tx_desc);
>> > + desc->wb_addr = cpu_to_be32(phys);
>>
>> One detail: since you don't have cache-coherent DMA, "desc" will
>> reside in uncached memory, so you try to minimize the number of accesses.
>> It's probably faster if you build the descriptor on the stack and
>> then atomically copy it over, rather than assigning each member at
>> a time.
>
> DMA coherent memory is write combining, so multiple writes will be
> coalesced. This also means that barriers may be required to ensure the
> descriptors are pushed out in a timely manner if something like writel()
> is not used in the transmit-triggering path.
>
Currently writel is used in xmit.
And regmap_write -> writel is used in poll.
Thanks
--
To unsubscribe from this list: send the line "unsubscribe devicetree" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
