Hello Helin, On Thu, Sep 18, 2014 at 4:39 AM, Zhang, Helin <helin.zhang at intel.com> wrote:
> Hi David > > > > *From:* David Marchand [mailto:david.marchand at 6wind.com] > *Sent:* Wednesday, September 17, 2014 10:03 PM > *To:* Zhang, Helin > *Cc:* dev at 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 at 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
