Re: [PATCH v7 1/4] spinlock: A new lockref structure for locklessupdate of refcount

[Linus Torvalds]

On Fri, Aug 30, 2013 at 2:27 AM, Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
>
> Here on Ubuntu/precise v12.04.3 AMD64 I get these numbers for total loops:
>
> lockref:  w/o patch | w/ patch
> ======================
> Run #1: 2.688.094 | 2.643.004
> Run #2: 2.678.884 | 2.652.787
> Run #3: 2.686.450 | 2.650.142
> Run #4: 2.688.435 | 2.648.409
> Run #5: 2.693.770 | 2.651.514

Yes, so this is pretty much expected.

If you don't have a very high core count (you don't mention your
system, but that's pretty - I get ~65 million repetitions in 10
seconds on my i5-670), the cmpxchg will not help - because you don't
actually see the bad "wait on spinlock" behavior in the first place.

And a "cmpxchg" is slightly slower than the very optimized spinlocks,
and has that annoying "read original value" first issue too. So the
patch can make things a bit slower, although it will depends on the
microarchitecture (and as mentioned elsewhere, there are other things
that can make a bigger difference boot-to-boot - dentry allocation
details etc can have "sticky" performance impact).

So we may take a small hit in order to then *not* have horrible
scalability at the high end.

               Linus
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