On Sun, Sep 18, 2011 at 7:48 PM, Arnaud Lacombe <lacom...@gmail.com> wrote:
> Hi, > > On Sun, Sep 18, 2011 at 10:01 PM, Luigi Rizzo <ri...@iet.unipi.it> wrote: > > On Sun, Sep 18, 2011 at 06:05:33PM -0400, Arnaud Lacombe wrote: > >> Hi, > >> > >> On Sun, Sep 18, 2011 at 5:06 PM, Luigi Rizzo <ri...@iet.unipi.it> > wrote: > >> > On Sun, Sep 18, 2011 at 03:19:46PM -0400, Arnaud Lacombe wrote: > >> >> Hi, > >> >> > >> >> On Sat, Sep 17, 2011 at 4:32 PM, YongHyeon PYUN <pyu...@gmail.com> > wrote: > >> >> > On Sat, Sep 17, 2011 at 11:57:10AM +0430, Hooman Fazaeli wrote: > >> >> >> Hi list, > >> >> >> > >> >> >> The data sheet for intel 82576 advertises IP TX/RX checksum > offload > >> >> >> but the driver does not set CSUM_IP in ifp->if_hwassist. Does this > mean that > >> >> >> driver (and chip) do not support IP TX checksum offload or the > support for > >> >> >> TX is not yet included in the driver? > >> > ... > >> >> This is slightly off-topic, but still.. > >> >> > >> >> FWIW, I'm not really impressed by what chips claim to support vs. > what > >> >> has been implemented in the driver. As per the product brief, the > >> > ... > >> >> [0]: the commit message say "performance was not good", but it is not > >> >> the driver's developer to decide whether or not a feature is good or > >> >> not. The developer's job is to implement the chip capabilities, and > >> >> let it to the user to enable or disable the capabilities. At best, > the > >> >> developer can decide whether or not to enable the feature by default. > >> > > >> > actually, this is a perfect example where the developer has done the > >> > right thing: implemented the feature, verified that performance is > bad, > >> > hence presumably removed support for the feature from the code (which > also > >> > means that the normal code path will run faster because there are no > >> > run-time decisions to be made). > >> > > >> > "optional" features are often costly even when disabled. > >> > > >> I forgot to mention that in this case, the code full of > >> EM_MULTIQUEUE's #ifdef and shared code is still fully compatible with > >> the multiqueue's architecture. The only thing removed is a conditional > >> and an assignation in the driver's attachment which was enabling the > >> feature, ie. the cost you point out is still paid today, without any > >> benefit. > > > > the above suggests that you have a wonderful opportunity: with just > > a little bit of time and effort you should be able to complete/re-enable > > the missing code, run tests that you believe significant (given > > your statement below) and prove or disprove the comment about > > performance. > > > Which I did about a week ago, to finally discover that the NIC only > had only 3 MSI-X vectors configured in its EEPROM[0], and thus the > MSI-X PCI capability field ends up also with being assigned with those > 3 vectors. However, the 82574 datasheet clearly say that up-to 5 > vector can be configured, but I obviously did not find the magic trick > to make it so. Maybe I'll find some time and try to reprogram the > EEPROM. Beside that, it was clear that the old multiqueue did not > support only 3 vector being available and thus fell back on MSI. It is > not clear in jfv@'s comment whether he really tested multiqueue, or > did he test the fall-back MSI mode. > > As the PCI spec is not public, I've not been able to find out from the > few public datasheet how the PCI MSI-X capability field is first > programmed. I'd assume that the BIOS is using the data in the NVM to > program it at power up. > > - Arnaud > > [0]: at least, the MSI_X_NUM field of the NVM at offset 0x1b is 2, > thus 3 vectors. > I give answers to those who treat me with respect, I view them as collaborators, we improve the drivers for everyone's benefit, rather than jumping in to throw a critical remark here, a negative innuendo there... If you notice, the Linux driver did not enable multiqueue on the hardware either, so do you think a whole department of software engineers backed by the hardware engineers who designed the damn thing might have had a reason? IN FACT, as I have a bit more freedom with FreeBSD, I went ahead and tried it for a while just because I could, implementing the code was not difficult. Over time however that code proved to be a source of instability and thus was disabled. I have heard a rumor that the Linux crew may actually be trying a second time to make it work, and that might give me cause to look at it again too, but its not clear if I'll have time with other priorities. Jack _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
