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

[Weijie Yang]

On Tue, May 6, 2014 at 6:22 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> On Mon, 2014-05-05 at 13:46 -0700, Andrew Morton wrote:
>> On Mon, 05 May 2014 11:00:44 -0700 Davidlohr Bueso <davidlohr@xxxxxx> wrote:
>>
>> > > > @@ -339,12 +338,14 @@ static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
>> > > >         unsigned long handle;
>> > > >         u16 size;
>> > > >
>> > > > -       read_lock(&meta->tb_lock);
>> > > > +       while(atomic_cmpxchg(&meta->table[index].state, IDLE, ACCESS) != IDLE)
>> > > > +               cpu_relax();
>> > > > +
>> > >
>> > > So... this might be dumb question, but this looks like a spinlock
>> > > implementation.
>> > >
>> > > What advantage does this have over a standard spinlock?
>> >
>> > I was wondering the same thing. Furthermore by doing this you'll loose
>> > the benefits of sharing the lock... your numbers do indicate that it is
>> > for the better. Also, note that hopefully rwlock_t will soon be updated
>> > to be fair and perform up to par with spinlocks, something which is long
>> > overdue. So you could reduce the critical region by implementing the
>> > same granularity, just don't implement your own locking schemes, like
>> > this.

Actually, the main reason I use a CAS rather than a standard lock here is
that I want to minimize the meta table memory overhead. A tiny reason is
my fuzzy memory that CAS is more efficient than spinlock (please correct me
if I am wrong).

Anyway, I changed the CAS to spinlock and rwlock, re-test them:

      Test       lock-free	   spinlock     rwlock
------------------------------------------------------
 Initial write   1424141.62   1426372.84   1423019.21
       Rewrite   1652504.81   1623307.14   1653682.04
          Read  11404668.35  11242885.05  10938125.00
       Re-read  11555483.75   11253906.6  10837773.50
  Reverse Read   8394478.17   8277250.34   7768057.39
   Stride read   9372229.95   9010498.53   8692871.77
   Random read   9187221.90   8988080.55   8661184.60
Mixed workload   5843370.85   5414729.54   5451055.03
  Random write   1608947.04   1572276.64   1588866.51
        Pwrite   1311055.32   1302463.04   1302001.06
         Pread   4652056.11   4555802.18   4469672.34

And I cann't say which one is the best, they have the similar performance.

Wait, iozone will create temporary files for every test thread, so there is no
possibility that these threads access the same table[index] concurrenctly.
So, I use fio to test the raw zram block device.
To enhance the possibility of access the same table[index] conflictly, I set zram
with a small disksize(10M) and let thread run with large loop count.

On the same test machine, the fio test command is:
fio --bs=32k --randrepeat=1 --randseed=100 --refill_buffers
--scramble_buffers=1 --direct=1 --loops=3000 --numjobs=4
--filename=/dev/zram0 --name=seq-write --rw=write --stonewall
--name=seq-read --rw=read --stonewall --name=seq-readwrite
--rw=rw --stonewall --name=rand-readwrite --rw=randrw --stonewall

    Test      base    lock-free   spinlock   rwlock
------------------------------------------------------
seq-write   935109.2   999580.5   998134.8   994384.6
 seq-read  5598064.6  6444011.5  6243184.6  6197514.2
   seq-rw  1403963.0  1635673.0  1633823.0  1635972.2
  rand-rw  1389864.4  1612520.4  1613403.6  1612129.8

This result(KB/s, average of 5 tests) shows the performance improvement
on base version, however, I cann't say which method is the best.

>>
>> It sounds like seqlocks will match this access pattern pretty well?
>
> Indeed. And after a closer look, except for zram_slot_free_notify(),
> that lock is always shared. So, unless fine graining it implies taking
> the lock exclusively like in this patch (if so, that needs to be
> explicitly documented in the changelog), we would ideally continue to
> share it. That _should_ provide nicer performance numbers when using the
> correct lock.
>

Andrew mentioned seqlocks, however, I think it is hard the use seqlocks here
after I recheck the codes. No matter use it as a meta global lock or a
table[index] lock. The main reason is the writer will free the handle rather than
just change some value.
If I misunderstand you, please let me know.

Now, I am in a delimma. For minimizing the memory overhead, I like to use CAS.
However, it is not a standard way.

Any complaint or suggestions are welcomed.

Regards,

>
>


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