Re: [PATCH] zram: remove global tb_lock by using lock-free CAS

[Minchan Kim]

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@xxxxxxxxxx> 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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