Subject: [linux-audio-dev] Re: Scheduler ( was: Just a second ) ...
From: Linus Torvalds (torvalds_AT_transmeta.com)
Date: Tue Dec 18 2001 - 19:16:48 EET
On Tue, 18 Dec 2001, Roger Larsson wrote:
>
> Lets see: we have >1 GHz CPU and interrupts at >1000 Hz
> => 1 Mcycle / interrupt - is that insane?
Ehh.. First off, the CPU may be 1GHz, but the memory subsystem, and the
PCI subsystem definitely are _not_. Most PCI cards still run at a
(comparatively) leisurely 33MHz, and when we're talking about audio, we're
talking about actually having to _access_ that audio device.
Yes. At 33MHz, not at 1GHz.
Also, at 32-byte fragments, the frequency is actually 5.5kHz, not 1kHz.
Now, I seriously doubt the mp3-player actually used 32-byte fragments (it
probably just asked for something small, and got it), but let's say it
asked for something in the kHz range (ie 256-512 byte frags). That does
_not_ equate to "1 Mcycle". It equates to 33 _kilocycles_ in PCI-land, and
a PCI read will take several cycles.
> If the hardware can support it? Why not let it? It is really up to the
> applications/user to decide...
Well, this particular user was unhappy with the CPU spending a noticeably
amount of time on just web-surfing and mp3-playing.
So clearly the _user_ didn't ask for it.
And I suspect that the app writer just didn't even realize what he did. He
may have used another sound card that didn't even allow small fragments.
Linus
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