Re: MTU on a virtio-net device?
Michael Tokarev wrote:
Dor Laor wrote:
Michael Tokarev wrote:
Dor Laor wrote:
Michael Tokarev wrote:
Right now (2.6.27), there's no way to change MTU of a
virtio-net interface, since the mtu-changing method is
not provided. Is there a simple way to add such a
beast?
It should be a nice easy patch for mtu < 4k.
You can just implement a 'change_mtu' handler like:
[]
Well, this isn't enough I think. That is, new_mtu's upper cap should be
less than PAGE_SIZE due to various additional data structures. But it
is enough to start playing.
The virtio header is in a separate ring entry so no prob.
virtio header is one thing. Ethernet frame is another. And
so on. From the last experiment (sending 2000bytes-payload
pings resulting in 2008 bytes total, and 528 bytes missing
with original mtu=1500), it seems like the necessary upper
cap is PAGE_SIZE-28. Or something similar.
Also see receive_skb() routine:
receive_skb(struct net_device *dev, struct sk_buff *skb, unsigned len)
{
if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
/*drop*/
}
len -= sizeof(struct virtio_net_hdr);
if (len <= MAX_PACKET_LEN) {
...
So it seems that virtio_net_hdr is in here, just like
ethernet header.
[]
So something else has to be changed for this to work, it seems.
You're right, this was needs to be changed to:
/* FIXME: MTU in config. */
#define MAX_PACKET_LEN (ETH_HLEN+ETH_DATA_LEN)
You can change it to PAGE_SIZE or have the current mtu.
so s/MAX_PACKET_LEN/dev->mtu/g for the whole driver, it
seems. Plus/minus sizeof(virtio_net_hdr) - checking this now.
This constant is used in 3 places:
receive_skb(): if (len <= MAX_PACKET_LEN) {
(this one seems to be wrong, but again I don't know much
internals of all this stuff)
here, dev->mtu is what we want.
try_fill_recv(): skb = netdev_alloc_skb(vi->dev, MAX_PACKET_LEN);
here, we don't have dev, but have vi->dev, should be ok too.
try_fill_recv(): skb_put(skb, MAX_PACKET_LEN);
ditto
I was too lazy to write a complete patch.
And by the way, what is "big_packets" here?
It's a bit harder here, IIRC qemu also has a 4k limit.
Not that it can be done in a short period.
Anyway you can use GSO and achieve similar performance.
Ok, so I changed MAX_PACKET_LEN to be PAGE_SIZE (current MTU
seems to be more appropriate but PAGE_SIZE is enough for
testing anyway). It seems to be working, and network
speed increased significantly with MTU=3500 compared with
former 1500 - it seems it's about 2 times faster (which is
quite expectable, since there's 2x less context switches,
transmissions and the like).
I'm asking because I'm not familiar with the internals,
Still... ;)
Thanks!
/mjt
You seems to be a fast learner :)
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Re: MTU on a virtio-net device?
Dor Laor wrote:
> Michael Tokarev wrote:
>> Dor Laor wrote:
>>
>>> Michael Tokarev wrote:
>>>
Right now (2.6.27), there's no way to change MTU of a
virtio-net interface, since the mtu-changing method is
not provided. Is there a simple way to add such a
beast?
>>> It should be a nice easy patch for mtu < 4k.
>>> You can just implement a 'change_mtu' handler like:
[]
>> Well, this isn't enough I think. That is, new_mtu's upper cap should be
>> less than PAGE_SIZE due to various additional data structures. But it
>> is enough to start playing.
>>
> The virtio header is in a separate ring entry so no prob.
virtio header is one thing. Ethernet frame is another. And
so on. From the last experiment (sending 2000bytes-payload
pings resulting in 2008 bytes total, and 528 bytes missing
with original mtu=1500), it seems like the necessary upper
cap is PAGE_SIZE-28. Or something similar.
Also see receive_skb() routine:
receive_skb(struct net_device *dev, struct sk_buff *skb, unsigned len)
{
if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
/*drop*/
}
len -= sizeof(struct virtio_net_hdr);
if (len <= MAX_PACKET_LEN) {
...
So it seems that virtio_net_hdr is in here, just like
ethernet header.
[]
>> So something else has to be changed for this to work, it seems.
> You're right, this was needs to be changed to:
> /* FIXME: MTU in config. */
> #define MAX_PACKET_LEN (ETH_HLEN+ETH_DATA_LEN)
>
> You can change it to PAGE_SIZE or have the current mtu.
so s/MAX_PACKET_LEN/dev->mtu/g for the whole driver, it
seems. Plus/minus sizeof(virtio_net_hdr) - checking this now.
This constant is used in 3 places:
receive_skb(): if (len <= MAX_PACKET_LEN) {
(this one seems to be wrong, but again I don't know much
internals of all this stuff)
here, dev->mtu is what we want.
try_fill_recv(): skb = netdev_alloc_skb(vi->dev, MAX_PACKET_LEN);
here, we don't have dev, but have vi->dev, should be ok too.
try_fill_recv(): skb_put(skb, MAX_PACKET_LEN);
ditto
And by the way, what is "big_packets" here?
Ok, so I changed MAX_PACKET_LEN to be PAGE_SIZE (current MTU
seems to be more appropriate but PAGE_SIZE is enough for
testing anyway). It seems to be working, and network
speed increased significantly with MTU=3500 compared with
former 1500 - it seems it's about 2 times faster (which is
quite expectable, since there's 2x less context switches,
transmissions and the like).
I'm asking because I'm not familiar with the internals,
Still... ;)
Thanks!
/mjt
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Re: MTU on a virtio-net device?
Dor Laor wrote:
> Michael Tokarev wrote:
>> Right now (2.6.27), there's no way to change MTU of a
>> virtio-net interface, since the mtu-changing method is
>> not provided. Is there a simple way to add such a
>> beast?
>>
> It should be a nice easy patch for mtu < 4k.
> You can just implement a 'change_mtu' handler like:
>
> static int virtio_change_mtu(struct net_device *netdev, int new_mtu)
> {
>if(new_mtu < ETH_ZLEN || new_mtu > PAGE_SIZE)
>return -EINVAL;
>netdev->mtu = new_mtu;
>return 0;
> }
Well, this isn't enough I think. That is, new_mtu's upper cap should be
less than PAGE_SIZE due to various additional data structures. But it
is enough to start playing.
I just added the above method, which allowed me to set MTU to 3500
(arbitrary). But it still does not work. In guest, I see the
following while pinging it from host with `ping -s2000':
16:26:57.952684 IP truncated-ip - 528 bytes missing! 81.13.33.145 >
81.13.33.150: ICMP echo request, id 12869, seq 19, length 2008
16:26:58.954133 IP truncated-ip - 528 bytes missing! 81.13.33.145 >
81.13.33.150: ICMP echo request, id 12869, seq 20, length 2008
...
So something else has to be changed for this to work, it seems.
That's why I wrote:
>> I'm asking because I'm not familiar with the internals,
[...]
;)
Thanks!
/mjt
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Re: MTU on a virtio-net device?
Michael Tokarev wrote:
Right now (2.6.27), there's no way to change MTU of a
virtio-net interface, since the mtu-changing method is
not provided. Is there a simple way to add such a
beast?
It should be a nice easy patch for mtu < 4k.
You can just implement a 'change_mtu' handler like:
static int virtio_change_mtu(struct net_device *netdev, int new_mtu)
{
if(new_mtu < ETH_ZLEN || new_mtu > PAGE_SIZE)
return -EINVAL;
netdev->mtu = new_mtu;
return 0;
}
I'm asking because I'm not familiar with the internals,
and because, I think, increasing MTU (so that the
resulting skb still fits in a single page) will increase
performance significantly, at least on a internal/virtual
network -- currently there are just way too many context
switches and the like while copying data from one guest
to another or between guest and host.
Thanks!
/mjt
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MTU on a virtio-net device?
Right now (2.6.27), there's no way to change MTU of a virtio-net interface, since the mtu-changing method is not provided. Is there a simple way to add such a beast? I'm asking because I'm not familiar with the internals, and because, I think, increasing MTU (so that the resulting skb still fits in a single page) will increase performance significantly, at least on a internal/virtual network -- currently there are just way too many context switches and the like while copying data from one guest to another or between guest and host. Thanks! /mjt -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
