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SubjectRe: [PATCH] zram: remove global tb_lock by using lock-free CAS
Hello David,

On Mon, May 12, 2014 at 07:49:18AM -0700, Davidlohr Bueso wrote:
> On Mon, 2014-05-12 at 14:15 +0900, Minchan Kim wrote:
> > On Sat, May 10, 2014 at 02:10:08PM +0800, Weijie Yang wrote:
> > > On Thu, May 8, 2014 at 2:24 PM, Minchan Kim <minchan@kernel.org> wrote:
> > > > On Wed, May 07, 2014 at 11:52:59PM +0900, Joonsoo Kim wrote:
> > > >> >> Most popular use of zram is the in-memory swap for small embedded system
> > > >> >> so I don't want to increase memory footprint without good reason although
> > > >> >> it makes synthetic benchmark. Alhought it's 1M for 1G, it isn't small if we
> > > >> >> consider compression ratio and real free memory after boot
> > > >>
> > > >> We can use bit spin lock and this would not increase memory footprint for 32 bit
> > > >> platform.
> > > >
> > > > Sounds like a idea.
> > > > Weijie, Do you mind testing with bit spin lock?
> > >
> > > Yes, I re-test them.
> > > This time, I test each case 10 times, and take the average(KS/s).
> > > (the test machine and method are same like previous mail's)
> > >
> > > Iozone test result:
> > >
> > > Test BASE CAS spinlock rwlock bit_spinlock
> > > --------------------------------------------------------------
> > > Initial write 1381094 1425435 1422860 1423075 1421521
> > > Rewrite 1529479 1641199 1668762 1672855 1654910
> > > Read 8468009 11324979 11305569 11117273 10997202
> > > Re-read 8467476 11260914 11248059 11145336 10906486
> > > Reverse Read 6821393 8106334 8282174 8279195 8109186
> > > Stride read 7191093 8994306 9153982 8961224 9004434
> > > Random read 7156353 8957932 9167098 8980465 8940476
> > > Mixed workload 4172747 5680814 5927825 5489578 5972253
> > > Random write 1483044 1605588 1594329 1600453 1596010
> > > Pwrite 1276644 1303108 1311612 1314228 1300960
> > > Pread 4324337 4632869 4618386 4457870 4500166
> > >
> > > Fio test result:
> > >
> > > Test base CAS spinlock rwlock bit_spinlock
> > > -------------------------------------------------------------
> > > seq-write 933789 999357 1003298 995961 1001958
> > > seq-read 5634130 6577930 6380861 6243912 6230006
> > > seq-rw 1405687 1638117 1640256 1633903 1634459
> > > rand-rw 1386119 1614664 1617211 1609267 1612471
> > >
> > >
> > > The base is v3.15.0-rc3, the others are per-meta entry lock.
> > > Every optimization method shows higher performance than the base, however,
> > > it is hard to say which method is the most appropriate.
> >
> > It's not too big between CAS and bit_spinlock so I prefer general method.
>
> Well, I imagine that's because the test system is small enough that the
> lock is not stressed enough. Bit spinlocks are considerably slower than
> other types. I'm not sure if we really care for the case of zram, but in
> general I really dislike this lock. It suffers from just about
> everything our regular spinlocks try to optimize, specially unfairness
> in who gets the lock when contended (ticketing).

But as you said, in general, you're right but it's not the case for zram.
Most popular zram usecase is in-memory swap for small embedded system(at most,
4 CPU, even, they don't turn on always) so I believe lock contention
(concurrent swapout of same slot? concurrent swapread of same slot)
is too much rare(ie, actually it wouldn't happen by upper layer's lock).

Another usecase zram-blk, yeb, thesedays, some guys start to use zram as block
device but it would be same with zram-swap because upper layer(ex, file system)
would already have a lock to prevent concurrent access of the block so
contention would be rare, too.

I don't want to bloat zram's memory footprint for minor usecase, even, without
real report with the number. We have reasonable rationale to use bit_spin_lock
like above.

>
> Thanks,
> Davidlohr
>
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--
Kind regards,
Minchan Kim


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