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?
--
Kind regards,
Minchan Kim
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