CPU underload

[Chris H]

On Mon, 9 Nov 2015 20:09:30 -0700 Warner Losh <imp at bsdimp.com> wrote

> > On Nov 9, 2015, at 10:46 AM, Ian Lepore <ian at FreeBSD.org> wrote:
> > 
> > On Tue, 2015-11-10 at 00:42 +0700, Eugene Grosbein wrote:
> >> On 10.11.2015 00:39, Ian Lepore wrote:
> >>> On Sun, 2015-11-08 at 11:23 -0800, Adrian Chadd wrote:
> >>>> ok, what's the l1 cache size reported at boot up?
> >>>> 
> >>>> I think I may just bump them all to 64.
> >>> 
> >>> 64 is not some kind of magic panacea.  The value needs to be set to
> >>> the
> >>> cache line size for the runtime platform.  If the right value is
> >>> 32,
> >>> then setting it to 64 will just waste memory.
> 
> If we ever have a universal kernel, 64 could run on either 32 or
> 64 byte cache lines. Since we don’t, this is largely correct. It was
> mostly a quick and easy to test suggestion, it doesn’t matter since:
> 
> >>>> L1 d-cache: 4 ways of 256 sets, 32 bytes per line
> 
> >> Is it for instruction cache or for data cache?
> > 
> > Only the data cache size matters for USB_HOST_ALIGN.
> 
> Since we’re flushing the data cache, and we don’t want the
> device visible part of the USB buffers to share a cache line
> with other data for the host to use about the device. Making
> it too big doesn’t help, and costs memory. Making it too small
> is fatal: usb simply won’t work.
Pitty. Cause it seems like 48 might have been a nice compromise. :)
> 
> Why we don’t have a panic when the cache line size is larger
> than usb_host_align, and a warning when it is smaller is
> beyond me...
> 
> Warner