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SubjectRe: question wrt context switching during disk i/o
On Sat, 21 Oct 2000, Mike Galbraith wrote:

> On Fri, 20 Oct 2000, Mark Hahn wrote:
>
> > > This is something that has been bugging me for a while. I notice
> > > on my system that during disk write we do much context switching,
> > > but not during disk read. Why is that?
> >
> > bdflush is broken in current kernels. I posted to linux-mm about this,
> > but Rik et al haven't shown any interest. I normally see bursts of
> > up to around 40K cs/second when doing writes; I hacked a little
> > premption counter into the kernel and verified that they're practially
> > all bdflush...
>
> P.S.
>
> I took a ktrace snapshot of iozone doing 8k writes. This seems like
> a strange and expensive way to only unplug a device ;-) Anyone know
> why?
>
> c010a976 system_call +<22/40> (0.16) pid(257)
> c0131bc0 sys_write +<10/d8> (0.12) pid(257)
> ...
> c010a99a ret_from_sys_call +<6/19> (0.20) pid(257)
> c0117f7b schedule +<13/404> (2.14) pid(257->5)
> c01091a3 __switch_to +<13/cc> (1.16) pid(5)
> c01888c6 generic_unplug_device +<e/38> (0.27) pid(5)
> c0117f7b schedule +<13/404> (0.51) pid(5->257)
> c01091a3 __switch_to +<13/cc> (0.30) pid(257)
> c010a99a ret_from_sys_call +<6/19> (1.20) pid(257)

(arg!) P.P.S

That was perhaps too brief to be clear exactly what I mean.

In a trace segment 263 milliseconds long we switch to kflushd
279 times. 251 of 279 cases are exactly the above.

-Mike

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