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

[Minchan Kim <minchan@kernel.org>]

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.

> 
> To bit_spinlock, the modified code is mainly like this:
> 
> +#define ZRAM_FLAG_SHIFT 16
> +
> enum zram_pageflags {
>  	/* Page consists entirely of zeros */
> -	ZRAM_ZERO,
> +	ZRAM_ZERO = ZRAM_FLAG_SHIFT + 1,
> +	ZRAM_ACCESS,
>  
>  	__NR_ZRAM_PAGEFLAGS,
>  };
>  
>  /* Allocated for each disk page */
>  struct table {
>  	unsigned long handle;
> -	u16 size;	/* object size (excluding header) */
> -	u8 flags;
> +	unsigned long value;

Why does we need to change flags and size "unsigned long value"?
Couldn't we use existing flags with just adding new ZRAM_TABLE_LOCK?


>  } __aligned(4);
> 
> The lower ZRAM_FLAG_SHIFT bits of table.value is size, the higher bits
> is for zram_pageflags. By this means, it doesn't increase any memory
> overhead on both 32-bit and 64-bit system.
> 
> Any complaint or suggestions are welcomed.

Anyway, I'd like to go this way.
Pz, resend formal patch with a number.

Thanks!

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

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