ix(intel) vs mlxen(mellanox) 10Gb performance

[Daniel Braniss]

> On 22 Aug 2015, at 14:59, Rick Macklem <rmacklem at uoguelph.ca> wrote:
> 
> Daniel Braniss wrote:
>> 
>>> On Aug 22, 2015, at 12:46 AM, Rick Macklem <rmacklem at uoguelph.ca> wrote:
>>> 
>>> Yonghyeon PYUN wrote:
>>>> On Wed, Aug 19, 2015 at 09:00:35AM -0400, Rick Macklem wrote:
>>>>> Hans Petter Selasky wrote:
>>>>>> On 08/19/15 09:42, Yonghyeon PYUN wrote:
>>>>>>> On Wed, Aug 19, 2015 at 09:00:52AM +0200, Hans Petter Selasky wrote:
>>>>>>>> On 08/18/15 23:54, Rick Macklem wrote:
>>>>>>>>> Ouch! Yes, I now see that the code that counts the # of mbufs is
>>>>>>>>> before
>>>>>>>>> the
>>>>>>>>> code that adds the tcp/ip header mbuf.
>>>>>>>>> 
>>>>>>>>> In my opinion, this should be fixed by setting if_hw_tsomaxsegcount
>>>>>>>>> to
>>>>>>>>> whatever
>>>>>>>>> the driver provides - 1. It is not the driver's responsibility to
>>>>>>>>> know if
>>>>>>>>> a tcp/ip
>>>>>>>>> header mbuf will be added and is a lot less confusing that expecting
>>>>>>>>> the
>>>>>>>>> driver
>>>>>>>>> author to know to subtract one. (I had mistakenly thought that
>>>>>>>>> tcp_output() had
>>>>>>>>> added the tc/ip header mbuf before the loop that counts mbufs in the
>>>>>>>>> list.
>>>>>>>>> Btw,
>>>>>>>>> this tcp/ip header mbuf also has leading space for the MAC layer
>>>>>>>>> header.)
>>>>>>>>> 
>>>>>>>> 
>>>>>>>> Hi Rick,
>>>>>>>> 
>>>>>>>> Your question is good. With the Mellanox hardware we have separate
>>>>>>>> so-called inline data space for the TCP/IP headers, so if the TCP
>>>>>>>> stack
>>>>>>>> subtracts something, then we would need to add something to the limit,
>>>>>>>> because then the scatter gather list is only used for the data part.
>>>>>>>> 
>>>>>>> 
>>>>>>> I think all drivers in tree don't subtract 1 for
>>>>>>> if_hw_tsomaxsegcount.  Probably touching Mellanox driver would be
>>>>>>> simpler than fixing all other drivers in tree.
>>>>>>> 
>>>>>>>> Maybe it can be controlled by some kind of flag, if all the three TSO
>>>>>>>> limits should include the TCP/IP/ethernet headers too. I'm pretty sure
>>>>>>>> we want both versions.
>>>>>>>> 
>>>>>>> 
>>>>>>> Hmm, I'm afraid it's already complex.  Drivers have to tell almost
>>>>>>> the same information to both bus_dma(9) and network stack.
>>>>>> 
>>>>>> Don't forget that not all drivers in the tree set the TSO limits before
>>>>>> if_attach(), so possibly the subtraction of one TSO fragment needs to go
>>>>>> into ip_output() ....
>>>>>> 
>>>>> Ok, I realized that some drivers may not know the answers before
>>>>> ether_ifattach(),
>>>>> due to the way they are configured/written (I saw the use of
>>>>> if_hw_tsomax_update()
>>>>> in the patch).
>>>> 
>>>> I was not able to find an interface that configures TSO parameters
>>>> after if_t conversion.  I'm under the impression
>>>> if_hw_tsomax_update() is not designed to use this way.  Probably we
>>>> need a better one?(CCed to Gleb).
>>>> 
>>>>> 
>>>>> If it is subtracted as a part of the assignment to if_hw_tsomaxsegcount
>>>>> in
>>>>> tcp_output()
>>>>> at line#791 in tcp_output() like the following, I don't think it should
>>>>> matter if the
>>>>> values are set before ether_ifattach()?
>>>>> 			/*
>>>>> 			 * Subtract 1 for the tcp/ip header mbuf that
>>>>> 			 * will be prepended to the mbuf chain in this
>>>>> 			 * function in the code below this block.
>>>>> 			 */
>>>>> 			if_hw_tsomaxsegcount = tp->t_tsomaxsegcount - 1;
>>>>> 
>>>>> I don't have a good solution for the case where a driver doesn't plan on
>>>>> using the
>>>>> tcp/ip header provided by tcp_output() except to say the driver can add
>>>>> one
>>>>> to the
>>>>> setting to compensate for that (and if they fail to do so, it still
>>>>> works,
>>>>> although
>>>>> somewhat suboptimally). When I now read the comment in sys/net/if_var.h
>>>>> it
>>>>> is clear
>>>>> what it means, but for some reason I didn't read it that way before? (I
>>>>> think it was
>>>>> the part that said the driver didn't have to subtract for the headers
>>>>> that
>>>>> confused me?)
>>>>> In any case, we need to try and come up with a clear definition of what
>>>>> they need to
>>>>> be set to.
>>>>> 
>>>>> I can now think of two ways to deal with this:
>>>>> 1 - Leave tcp_output() as is, but provide a macro for the device driver
>>>>> authors to use
>>>>>   that sets if_hw_tsomaxsegcount with a flag for "driver uses tcp/ip
>>>>>   header mbuf",
>>>>>   documenting that this flag should normally be true.
>>>>> OR
>>>>> 2 - Change tcp_output() as above, noting that this is a workaround for
>>>>> confusion w.r.t.
>>>>>   whether or not if_hw_tsomaxsegcount should include the tcp/ip header
>>>>>   mbuf and
>>>>>   update the comment in if_var.h to reflect this. Then drivers that
>>>>>   don't
>>>>>   use the
>>>>>   tcp/ip header mbuf can increase their value for if_hw_tsomaxsegcount
>>>>>   by
>>>>>   1.
>>>>>   (The comment should also mention that a value of 35 or greater is much
>>>>>   preferred to
>>>>>    32 if the hardware will support that.)
>>>>> 
>>>> 
>>>> Both works for me.  My preference is 2 just because it's very
>>>> common for most drivers that use tcp/ip header mbuf.
>>> Thanks for this comment. I tend to agree, both for the reason you state and
>>> also
>>> because the patch is simple enough that it might qualify as an errata for
>>> 10.2.
>>> 
>>> I am hoping Daniel Braniss will be able to test the patch and let us know
>>> if it
>>> improves performance with TSO enabled?
>> 
>> send me the patch and I’ll test it ASAP.
>> 	danny
>> 
> Patch is attached. The one for head will also include an update to the comment
> in sys/net/if_var.h, but that isn't needed for testing.


well, the plot thickens.

Yesterday, before running the new kernel, I decided to re run my test, and to my surprise
i was getting good numbers, about 300MGB/s with and without TSO.

this morning, the numbers were again bad, around 70MGB/s,what the ^%$#@!

so, after some coffee, I run some more tests, and some conclusions:
using a netapp(*) as the nfs client:
  - doing 
	ifconfig ix0 tso or -tso
    does some magic and numbers are back to normal - for a while

using another Fbsd/zfs as client all is nifty, actually a bit faster than the netapp (not a fair
comparison, since the zfs client is not heavily used) and I can’t see any degradation.
 
btw, this is with the patch applied, but was seeing similar numbers before the patch.

running with tso, initially I get around 300MGB/s, but after a while(sorry can’t be more scientific)
it drops down to about half,  and finally to a pathetic 70MGB/s

*: while running the tests I monitored the Netapp, and nothing out of the ordinary there.

cheers,
	danny