Re: [dpdk-dev] i40e: Steps and required configurations of how to achieve the best performance!

[David Marchand <david.marchand@6wind.com>]

Hello Helin,

On Thu, Sep 18, 2014 at 4:39 AM, Zhang, Helin <helin.zhang@intel.com> wrote:

>  Hi David
>
>
>
> *From:* David Marchand [mailto:david.marchand@6wind.com]
> *Sent:* Wednesday, September 17, 2014 10:03 PM
> *To:* Zhang, Helin
> *Cc:* dev@dpdk.org
> *Subject:* Re: [dpdk-dev] i40e: Steps and required configurations of how
> to achieve the best performance!
>
>
>
> On Wed, Sep 17, 2014 at 10:50 AM, Zhang, Helin <helin.zhang@intel.com>
> wrote:
>
>  For the ‘extended tag’, it was defined in PCIe spec, but actually not
> all BIOS implements it. Enabling it in BIOS or at runtime are two choices
> of doing the same thing. I don’t think it can be configured per PCI device
> in BIOS, so we don’t need to do that per PCI device in DPDK. Right?
> Actually we don’t want to touch PCIe settings in DPDK code, that’s why we
> want to let BIOS config as it is by default. If no better choice, we can do
> it in DPDK by changing configurations.
>
>
>
> - Ok, then if we can make a runtime decision (at dpdk level), there is no
> need for bios configuration and there is no need for a build option.
>
> Why don't we get rid of this option ?
>
>
>
> [Helin] Initially, we want to do that for BIOS, if specific BIOS does not
> implement it. That way it needs to be initialized once during
> initialization for each PCI device. Sure, that might not be the best
> option, but it is the easiest way. For Linux end users, the best option
> could be using ‘setpci’ command. It can enable ‘extended_tag’ per PCI
> device.
>

I am not sure I can see how easy it is since you are forcing this in a
build option.
Anyway, all this knowledge should be in the documentation and not in an
obscure build option that looks to be useless in the end.

The more I look at this, the more I think we did not have good enough
argument for this change in eal / igb_uio yet.

We have something that gives "better performance" on "some server" with
"some bios".



>
> As far as the per-device runtime configuration is concerned, I want to
> make sure this pci configuration will not break other "igb_uio" pci devices.
>
> If Intel can tell for sure this won't break other devices, then fine, we
> can go and enable this for all "igb_uio" pci devices.
>
>
>
> [Helin] It is in PCIe specification, and enable it can provide better
> performance generally. But I cannot confirm that it would not break any
> other devices, as I don’t validate all devices. If you really concern it,
> ‘setpci’ can be the best option for you. We can add a script for that later.
>

Why not a script, but documentation is important too: I would say that we
need an explicit list of platforms and nics which support this.



>
> - By the way, there is also the CONFIG_MAX_READ_REQUEST_SIZE option that
> seems to be disabled (or at least its value 0 seems to tell so).
>
> What is its purpose ?
>
>
>
> [Helin] Yes, it was added for performance tuning long long ago. But now it
> seems contribute nothing or too few for the performance number, so I just
> skip it. The default value does nothing on PCIe registers, just keep it as
> is.
>

Not so long ago to dpdk.org (somewhere around 1.7.0 ...).
If this code had no use for "so long", why did it end up on dpdk.org ?
Why should we keep it ?


Thanks.

-- 
David Marchand