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Posted by John Navas on 01/22/08 22:02
On Tue, 22 Jan 2008 21:42:49 +0000 (UTC), Ilya Zakharevich
<nospam-abuse@ilyaz.org> wrote in <fn5o0p$1fvj$1@agate.berkeley.edu>:
><spamfilter1@navasgroup.com>], who wrote in article <kdhap35jd7b50p197hbq7r30l57blmldpl@4ax.com>:
>> Bus overhead and latency.
>
>So far all this discussion is running around this circle:
> a) I ask: why do we observe bus overhead and latency;
> b) People answer: because of bus overhead and latency.
>Thanks, but no, thanks.
I (and others) have posted responsive answers, but you seem either
unwilling or unable to accept what I've posted, and repeatedly insulting
us does not encourage us to try harder.
One more (last) time: USB is a simple cheap bus, very dumb, without the
kind of priority interrupts and DMA transfers you're assuming. Overhead
is due to bus protocol signaling. Latency is due to having to wait
until the host (computer and controller) gets around to servicing you.
This is all covered in USB documentation, which I suggest you obtain and
study.
>> It's *polled* -- *not* priority interrupt-driven.
>
>Again: this is irrelevant as far as there is interrupt-driven wakeup call.
Not true.
>> Unlike USB, DMA transfers can proceed without polling.
>
>DMA transfers still require cooperation. Somebody should set up the
>buffers in time. (The difference, is, of course, in SIZE of the
>buffer. So it is quantitative, not qualitative.)
Not true.
>> The host computer stack has to compete with all other processes in the
>> host.
>
>This assumes that it is a process. If it is a device driver (as it
>would be on any sane OS), there would be no competition.
There is competition.
>> USB is cheap and simple (like hard disk PIO), and thus not terribly
>> efficient. You're making assumptions that just aren't valid. You
>> should read up on how it works.
>
>So I ask: if somebody knows how it works, please speak up, and people
>reply: you should read up on how it works.
>
>Thanks, but no, thanks.
Thanks, but no, I'm done. Think what you want.
--
Best regards,
John Navas
Panasonic DMC-FZ8 (and several others)
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