From: David Howells
> Sent: 15 September 2023 11:10
>
> David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> > > Add kunit tests to benchmark 256MiB copies to a UBUF iterator and an IOVEC
> > > iterator. This attaches a userspace VM with a mapped file in it
> > > temporarily to the test thread.
> >
> > Isn't that going to be completely dominated by the cache fills
> > from memory?
>
> Yes... but it should be consistent in the amount of time that consumes since
> no device drivers are involved. I can try adding the same folio to the
> anon_file multiple times - it might work especially if I don't put the pages
> on the LRU (if that's even possible) - but I wanted separate pages for the
> extraction test.
You could also just not do the copy!
Although you need (say) asm volatile("\n",:::"memory") to
stop it all being completely optimised away.
That might show up a difference in the 'out_of_line' test
where 15% on top on the data copies is massive - it may be
that the data cache behaviour is very different for the
two cases.
...
> > Some measurements can be made using readv() and writev()
> > on /dev/zero and /dev/null.
>
> Forget /dev/null; that doesn't actually engage any iteration code. The same
> for writing to /dev/zero. Reading from /dev/zero does its own iteration thing
> rather than using iterate_and_advance(), presumably because it checks for
> signals and resched.
Using /dev/null does exercise the 'copy iov from user' code.
Last time I looked at that the 32bit compat code was faster than
the 64bit code on x86!
David
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