Re: [PATCH v4] zsmalloc: merge size_class to reduce fragmentation

[Joonsoo Kim]

On Tue, Sep 30, 2014 at 08:10:22AM +0900, Minchan Kim wrote:
> Hey Joonsoo,
> 
> On Mon, Sep 29, 2014 at 04:45:27PM +0900, Joonsoo Kim wrote:
> > zsmalloc has many size_classes to reduce fragmentation and they are
> > in 16 bytes unit, for example, 16, 32, 48, etc., if PAGE_SIZE is 4096.
> > And, zsmalloc has constraint that each zspage has 4 pages at maximum.
> > 
> > In this situation, we can see interesting aspect.
> > Let's think about size_class for 1488, 1472, ..., 1376.
> > To prevent external fragmentation, they uses 4 pages per zspage and
> > so all they can contain 11 objects at maximum.
> > 
> > 16384 (4096 * 4) = 1488 * 11 + remains
> > 16384 (4096 * 4) = 1472 * 11 + remains
> > 16384 (4096 * 4) = ...
> > 16384 (4096 * 4) = 1376 * 11 + remains
> > 
> > It means that they have same characteristics and classification between
> > them isn't needed. If we use one size_class for them, we can reduce
> > fragementation and save some memory since both the 1488 and 1472 sized
> > classes can only fit 11 objects into 4 pages, and an object that's
> > 1472 bytes can fit into an object that's 1488 bytes, merging these
> > classes to always use objects that are 1488 bytes will reduce the total
> > number of size classes. And reducing the total number of size classes
> > reduces overall fragmentation, because a wider range of compressed pages
> > can fit into a single size class, leaving less unused objects in each
> > size class.
> > 
> > For this purpose, this patch implement size_class merging. If there is
> > size_class that have same pages_per_zspage and same number of objects
> > per zspage with previous size_class, we don't create new size_class.
> > Instead, we use previous, same characteristic size_class. With this way,
> > above example sizes (1488, 1472, ..., 1376) use just one size_class
> > so we can get much more memory utilization.
> > 
> > Below is result of my simple test.
> > 
> > TEST ENV: EXT4 on zram, mount with discard option
> > WORKLOAD: untar kernel source code, remove directory in descending order
> > in size. (drivers arch fs sound include net Documentation firmware
> > kernel tools)
> > 
> > Each line represents orig_data_size, compr_data_size, mem_used_total,
> > fragmentation overhead (mem_used - compr_data_size) and overhead ratio
> > (overhead to compr_data_size), respectively, after untar and remove
> > operation is executed.
> > 
> > * untar-nomerge.out
> > 
> > orig_size compr_size used_size overhead overhead_ratio
> > 525.88MB 199.16MB 210.23MB  11.08MB 5.56%
> > 288.32MB  97.43MB 105.63MB   8.20MB 8.41%
> > 177.32MB  61.12MB  69.40MB   8.28MB 13.55%
> > 146.47MB  47.32MB  56.10MB   8.78MB 18.55%
> > 124.16MB  38.85MB  48.41MB   9.55MB 24.58%
> > 103.93MB  31.68MB  40.93MB   9.25MB 29.21%
> >  84.34MB  22.86MB  32.72MB   9.86MB 43.13%
> >  66.87MB  14.83MB  23.83MB   9.00MB 60.70%
> >  60.67MB  11.11MB  18.60MB   7.49MB 67.48%
> >  55.86MB   8.83MB  16.61MB   7.77MB 88.03%
> >  53.32MB   8.01MB  15.32MB   7.31MB 91.24%
> > 
> > * untar-merge.out
> > 
> > orig_size compr_size used_size overhead overhead_ratio
> > 526.23MB 199.18MB 209.81MB  10.64MB 5.34%
> > 288.68MB  97.45MB 104.08MB   6.63MB 6.80%
> > 177.68MB  61.14MB  66.93MB   5.79MB 9.47%
> > 146.83MB  47.34MB  52.79MB   5.45MB 11.51%
> > 124.52MB  38.87MB  44.30MB   5.43MB 13.96%
> > 104.29MB  31.70MB  36.83MB   5.13MB 16.19%
> >  84.70MB  22.88MB  27.92MB   5.04MB 22.04%
> >  67.11MB  14.83MB  19.26MB   4.43MB 29.86%
> >  60.82MB  11.10MB  14.90MB   3.79MB 34.17%
> >  55.90MB   8.82MB  12.61MB   3.79MB 42.97%
> >  53.32MB   8.01MB  11.73MB   3.73MB 46.53%
> > 
> > As you can see above result, merged one has better utilization (overhead
> > ratio, 5th column) and uses less memory (mem_used_total, 3rd column).
> > 
> > Changes from v1:
> > - More commit description about what to do in this patch.
> > - Remove nr_obj in size_class, because it isn't need after initialization.
> > - Rename __size_class to size_class, size_class to merged_size_class.
> > - Add code comment for merged_size_class of struct zs_pool.
> > - Add code comment how merging works in zs_create_pool().
> > 
> > Changes from v2:
> > - Add more commit description (Dan)
> > - dynamically allocate size_class structure (Dan)
> > - rename objs_per_zspage to get_maxobj_per_zspage (Minchan)
> > 
> > Changes from v3:
> > - Add error handling logic in zs_create_pool (Dan)
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > ---
> >  mm/zsmalloc.c |   84 +++++++++++++++++++++++++++++++++++++++++++++++----------
> >  1 file changed, 70 insertions(+), 14 deletions(-)
> > 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index c4a9157..11556ae 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -187,6 +187,7 @@ enum fullness_group {
> >  static const int fullness_threshold_frac = 4;
> >  
> >  struct size_class {
> > +	int ref;
> 
> Couldn't we remove the ref from size_class by making zs_destroy_pool
> aware of merged size class like zs_create_pool?
> 

Hello,

I think that using ref would makes intuitive code. Although there is
some memory overhead, it is really small. So I prefer to this way.

But, if you think that removing ref is better, I will do it.
Please let me know your final decision.

Thanks.
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