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Subject: RE: [PATCH] clarify purpose of empty cache lines
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Date: Thu, 12 Oct 2023 08:07:37 +0200
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Thread-Topic: [PATCH] clarify purpose of empty cache lines
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From: =?iso-8859-1?Q?Morten_Br=F8rup?= <mb@smartsharesystems.com>
To: "Thomas Monjalon" <thomas@monjalon.net>
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> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Thursday, 12 October 2023 01.44
>=20
> 04/09/2023 10:43, Morten Br=F8rup:
> >  /** Force minimum cache line alignment. */
> >  #define __rte_cache_min_aligned =
__rte_aligned(RTE_CACHE_LINE_MIN_SIZE)
> >
> > +#define _RTE_CACHE_GUARD_HELPER2(unique) \
> > +		char cache_guard_ ## unique[RTE_CACHE_LINE_SIZE *
> RTE_CACHE_GUARD_LINES] \
> > +		__rte_cache_aligned
> > +#define _RTE_CACHE_GUARD_HELPER1(unique) =
_RTE_CACHE_GUARD_HELPER2(unique)
>=20
> What is the reason for this intermediate helper macro?
>=20
> > +/**
> > + * Empty cache lines, to guard against false sharing-like effects
> > + * on systems with a next-N-lines hardware prefetcher.
> > + *
> > + * Use as spacing between data accessed by different lcores,
> > + * to prevent cache thrashing on hardware with speculative =
prefetching.
> > + */
> > +#define RTE_CACHE_GUARD _RTE_CACHE_GUARD_HELPER1(__COUNTER__)

HELPER1 is required to convert __COUNTER__ to a number before HELPER2 =
concatenates it to cache_guard_.

If using HELPER2 directly,

#define RTE_CACHE_GUARD _RTE_CACHE_GUARD_HELPER2(__COUNTER__)

would expand to:

char cache_guard___COUNTER__[RTE_CACHE_LINE_SIZE * =
RTE_CACHE_GUARD_LINES]
	__rte_cache_aligned

Which is not unique, and would prevent using RTE_CACHE_GUARD multiple =
times in the same structure.