Date: Wed, 16 May 2001 15:22:51 -0700 (PDT) From: Matt Dillon <dillon@earth.backplane.com> To: Tor.Egge@fast.no Cc: arch@FreeBSD.ORG Subject: Re: on load control / process swapping Message-ID: <200105162222.f4GMMpC81247@earth.backplane.com> References: <200105162135.f4GLZdo78984@earth.backplane.com> <200105162211.AAA02889@midten.fast.no>
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:..
:> allow them to be reused in the next read(), and they will
:> already be in the L2 cache. If I don't free the underlying VM
:> pages the sequential read will force the L2 cache to cycle, and
:> I'll bet that is why you get such drastically different idle
:> times.
:
:Avoiding that copyout() is the major reason for increased idle time.
:
:The L2 cache will still cycle a lot with your suggested implementation
:for the load I used since the normal amount of outstanding IO is 25 MB
:(256 KB x 100). The L2 cache is a lot smaller then 25 MB.
:
:- Tor Egge
I'd have to see your test code. Doing a direct-read into a user buffer
has no cache impact at all (DMA does not go through the cpu cache).
If you are doing seek/read()s but not actually looking at the data that
is returned, your test results are going to be seriously skewed.
-Matt
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