On Wed, 29 Sep 2010 22:18:47 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote: > From: Eric Dumazet <dada1@xxxxxxxxxxxxx> > > last_ino was converted to an atomic variable to allow the inode_lock > to go away. However, contended atomics do not scale on large > machines, and new_inode() triggers excessive contention in such > situations. > > Solve this problem by providing to each cpu a per_cpu variable, > feeded by the shared last_ino, but once every 1024 allocations. > This reduces contention on the shared last_ino, and give same > spreading ino numbers than before (i.e. same wraparound after 2^32 > allocations). > > [npiggin: some extra commenting and use of defines] > > ... > > +#ifdef CONFIG_SMP > +#define LAST_INO_BATCH 1024 > +/* > + * Each cpu owns a range of LAST_INO_BATCH numbers. > + * 'shared_last_ino' is dirtied only once out of LAST_INO_BATCH allocations, > + * to renew the exhausted range. > + * > + * This does not significantly increase overflow rate because every CPU can > + * consume at most LAST_INO_BATCH-1 unused inode numbers. So there is > + * NR_CPUS*(LAST_INO_BATCH-1) wastage. At 4096 and 1024, this is ~0.1% of the > + * 2^32 range, and is a worst-case. Even a 50% wastage would only increase > + * overflow rate by 2x, which does not seem too significant. > + * > + * On a 32bit, non LFS stat() call, glibc will generate an EOVERFLOW > + * error if st_ino won't fit in target struct field. Use 32bit counter > + * here to attempt to avoid that. > + */ > +static DEFINE_PER_CPU(unsigned int, last_ino); > +static atomic_t shared_last_ino; > + > +static unsigned int last_ino_get(void) > +{ > + unsigned int *p = &get_cpu_var(last_ino); > + unsigned int res = *p; > + > + if (unlikely((res & (LAST_INO_BATCH-1)) == 0)) > + res = (unsigned int)atomic_add_return(LAST_INO_BATCH, > + &shared_last_ino) - LAST_INO_BATCH; May as well remove the "- LAST_INO_BATCH" there, I think. It'll skew the results a tad at startup, but why does that matter? > + *p = ++res; > + put_cpu_var(last_ino); > + return res; > +} > +#else > +static unsigned int last_ino_get(void) > +{ > + static unsigned int last_ino; > + > + return ++last_ino; > +} This is racy with CONFIG_PREEMPT on some architectures, I suspect. I'd suggest conversion to atomic_t with, of course, an explanatory comment ;) > +#endif -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html