Date: Thu, 8 Jul 2004 10:16:58 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Julian Elischer <julian@elischer.org> Cc: current@freebsd.org Subject: Re: speeding up ugen by an order of magnitude. Message-ID: <200407081716.i68HGwnU042339@apollo.backplane.com> References: <Pine.BSF.4.21.0407080108010.66234-100000@InterJet.elischer.org>
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I explored the ugen code after reading Julian's original posting,
and Bernd Walter's very informative postings. I also experimented
a bit with s10sh (which uses ugen to download from digital cameras).
Unfortunately my camera probably isn't the best test, and I didn't
get any huge improvement in performance... but it didn't break either,
and the 1K on-kernel-stack buffer definitely has to be fixed in any case.
My conclusion is that while a lot of copying occurs inside the kernel,
(in at least two places in fact), the overhead is inconsequential
relative to the 1 ms scan issue in the USB controller. Remember, this
is UGEN we are talking about here, not UMASS. It is not necessary to
squeeze out every last bit of performance, only to make the performance
'reasonable'.
The userland requests can certainly be made large enough (e.g. 64K,
128K, 1MB, whatever) (s10sh uses a 4KB ugen request, for example, but
can be easily increased). The 1K buf[] in UGEN seems to be most
responsible for screwing up performance. Increasing this buffer to
a reasonable size and ignoring the problem of data copying that occurs
in-kernel basically moves the entire problem into the xHCI device layer.
I would recommend that you guys simply increase the size of the buffer
in the UGEN layer for now, along the lines of Julian's patch or my patch,
and then work on making the xHCI layer deal with large requests more
efficiently as a separate stage. I email'd Julian the changes I made
in DragonFly which include a sysctl to allow the buffer size to be
set/changed on the fly. If you try to fix the half dozen or so
buffer copying / busdma issues as a pre-req, you will wind up getting
nothing done at all.
-Matt
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