On (04/26/17 15:08), Joonsoo Kim wrote:
> > > +struct zram_hash {
> > > + spinlock_t lock;
> > > + struct rb_root rb_root;
> > > };
> >
> > just a note.
> >
> > we can easily have N CPUs spinning on ->lock for __zram_dedup_get() lookup,
> > which can invole a potentially slow zcomp_decompress() [zlib, for example,
> > with 64k pages] and memcmp(). the larger PAGE_SHIFT is, the more serialized
> > IOs become. in theory, at least.
> >
> > CPU0 CPU1 ... CPUN
> >
> > __zram_bvec_write() __zram_bvec_write() __zram_bvec_write()
> > zram_dedup_find() zram_dedup_find() zram_dedup_find()
> > spin_lock(&hash->lock);
> > spin_lock(&hash->lock);
> > spin_lock(&hash->lock);
> > __zram_dedup_get()
> > zcomp_decompress()
> > ...
> >
> >
> > so may be there is a way to use read-write lock instead on spinlock for hash
> > and reduce write/read IO serialization.
>
> In fact, dedup release hash->lock before doing zcomp_decompress(). So,
> above contention cannot happen.
oh, my bad. you are right. somehow I didn't spot the unlock.
-ss
