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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