Re: [PATCH v2] lib: test bitmap vs IDA vs Maple Tree performance for region allocations

[Gary Guo]

On Sat Jul 11, 2026 at 7:36 AM BST, Yury Norov wrote:
> Compare the cost of allocating and freeing variable-sized regions using
> a bitmap, IDA and a Maple Tree. All implementations process the same
> randomly generated sequence of region sizes, ranging from 1 to 32 entries,
> until the configured capacity is exhausted.
>
> Run the benchmark at several capacities to show how the approaches
> scale. Report allocation and free times separately because bitmap,
> IDA and Maple Tree removal have substantially different costs.
>
> On x86/kvm, the output example is:
>
> type       alloc (ns)      free (ns)   capacity   memory (B)
> bitmap      179573071         342105    1000000       125000
> IDA          46555636       33931498    1000000       134864
> maple        18629665       19741396    1000000      1548304
> bitmap        1630912          30933     100000        12504
> IDA           6144785        3354590     100000        14288
> maple         1745026        1825032     100000       155408
> bitmap          28448           3374      10000         1256
> IDA            418978         333641      10000         1872
> maple          185398         211138      10000        15632
> bitmap           2253            610       1000          128
> IDA             42755          36432       1000          144
> maple           19728          23474       1000         1552
>
> Reported IDA and Maple Tree memory figures exclude slab overhead
> and transient allocations. The Maple Tree figure is additionally
> a lower-bound estimate that assumes fully occupied leaf nodes and
> excludes internal nodes.
>
> IDA has no region-allocation API, so each region is implemented as
> a sequence of single-ID allocations. The IDs remain contiguous
> because this benchmark fills an initially empty IDA monotonically.
>
> The benchmark is motivated by the discussion at the link below about
> choosing the best data structure for the channel ID pool with the
> capacity of 2048 IDs for the nova GPU driver.
>
> Specifically for 2048 IDs the result is:
>
> bitmap           3907            831       2048          256
> IDA             83545          71007       2048          848
> maple           34632          39710       2048         3600
>
> Link: https://lore.kernel.org/all/20260710-chid-maple-v1-1-4ee869055268@xxxxxxxxxx/
> Signed-off-by: Yury Norov <ynorov@xxxxxxxxxx>
> ---
> v2: add IDA benchmark (Matthew)
> v1: https://lore.kernel.org/all/20260711013910.349586-1-ynorov@xxxxxxxxxx/
>
>  MAINTAINERS                  |   3 +
>  lib/Kconfig.debug            |   9 ++
>  lib/Makefile                 |   1 +
>  lib/region_alloc_benchmark.c | 174 +++++++++++++++++++++++++++++++++++
>  4 files changed, 187 insertions(+)
>  create mode 100644 lib/region_alloc_benchmark.c
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 7cc4bca5a2c5..9e487a94aba4 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -4615,6 +4615,7 @@ F:	lib/bitmap.c
>  F:	lib/cpumask.c
>  F:	lib/find_bit.c
>  F:	lib/find_bit_benchmark.c
> +F:	lib/region_alloc_benchmark.c
>  F:	lib/test_bitmap.c
>  F:	lib/tests/cpumask_kunit.c
>  F:	tools/include/linux/bitfield.h
> @@ -15581,6 +15582,7 @@ F:	Documentation/core-api/maple_tree.rst
>  F:	include/linux/maple_tree.h
>  F:	include/trace/events/maple_tree.h
>  F:	lib/maple_tree.c
> +F:	lib/region_alloc_benchmark.c
>  F:	lib/test_maple_tree.c
>  F:	rust/helpers/maple_tree.c
>  F:	rust/kernel/maple_tree.rs
> @@ -29323,6 +29325,7 @@ F:	Documentation/core-api/xarray.rst
>  F:	include/linux/idr.h
>  F:	include/linux/xarray.h
>  F:	lib/idr.c
> +F:	lib/region_alloc_benchmark.c
>  F:	lib/test_xarray.c
>  F:	lib/xarray.c
>  F:	tools/testing/radix-tree
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 1244dcac2294..40872263cff1 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2683,6 +2683,15 @@ config FIND_BIT_BENCHMARK
>  
>  	  If unsure, say N.
>  
> +config REGION_ALLOC_BENCHMARK
> +	tristate "Benchmark bitmap, IDA and Maple Tree region allocation"
> +	help
> +	  This builds a microbenchmark comparing variable-sized region
> +	  allocation using bitmaps, IDA and Maple Tree. The benchmark
> +	  runs at module load time.
> +
> +	  If unsure, say N.
> +
>  config FIND_BIT_BENCHMARK_RUST
>  	tristate "Test find_bit functions in Rust"
>  	depends on RUST
> diff --git a/lib/Makefile b/lib/Makefile
> index 7f75cc6edf94..adb18810e3f7 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -64,6 +64,7 @@ obj-y += hexdump.o
>  obj-$(CONFIG_TEST_HEXDUMP) += test_hexdump.o
>  obj-y += kstrtox.o
>  obj-$(CONFIG_FIND_BIT_BENCHMARK) += find_bit_benchmark.o
> +obj-$(CONFIG_REGION_ALLOC_BENCHMARK) += region_alloc_benchmark.o
>  obj-$(CONFIG_FIND_BIT_BENCHMARK_RUST) += find_bit_benchmark_rust.o
>  obj-$(CONFIG_TEST_BPF) += test_bpf.o
>  test_dhry-objs := dhry_1.o dhry_2.o dhry_run.o
> diff --git a/lib/region_alloc_benchmark.c b/lib/region_alloc_benchmark.c
> new file mode 100644
> index 000000000000..0e000c4f9926
> --- /dev/null
> +++ b/lib/region_alloc_benchmark.c
> @@ -0,0 +1,174 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Benchmark bitmap, IDA and Maple Tree allocation of variable-sized regions. */
> +
> +#include <linux/bitmap.h>
> +#include <linux/idr.h>
> +#include <linux/kernel.h>
> +#include <linux/maple_tree.h>
> +#include <linux/module.h>
> +#include <linux/printk.h>
> +#include <linux/random.h>
> +#include <linux/xarray.h>
> +
> +#define MAP_SIZE	(1000000UL)
> +#define REGION_MAX_SIZE	32
> +
> +static DECLARE_BITMAP(alloc_bitmap, MAP_SIZE) __initdata;
> +/* One more request guarantees that even an all-ones trace reaches ENOSPC. */
> +static u8 region_sizes[MAP_SIZE + 1] __initdata;
> +static unsigned long region_indexes[MAP_SIZE] __initdata;
> +
> +static unsigned long __init benchmark_bitmap(unsigned long capacity)
> +{
> +	unsigned long count, index;
> +	ktime_t alloc_time, free_time;
> +	size_t memory;
> +
> +	bitmap_zero(alloc_bitmap, MAP_SIZE);
> +	alloc_time = ktime_get();

To eliminate possible effects of optimization messing up benchmark, please add a
barrier() after start timing and a barrier() before end timing. Probably you'd
want one between each loop iteration too.

> +	for (count = 0; count < ARRAY_SIZE(region_sizes); count++) {
> +		index = bitmap_find_next_zero_area(alloc_bitmap, capacity, 0,
> +						   region_sizes[count], 0);
> +		if (index >= capacity)
> +			break;
> +
> +		region_indexes[count] = index;
> +		bitmap_set(alloc_bitmap, index, region_sizes[count]);
> +	}
> +	alloc_time = ktime_get() - alloc_time;
> +
> +	index = count;
> +	free_time = ktime_get();
> +	while (index--)
> +		bitmap_clear(alloc_bitmap, region_indexes[index],
> +			     region_sizes[index]);
> +	free_time = ktime_get() - free_time;
> +	memory = BITS_TO_LONGS(capacity) * sizeof(unsigned long);
> +	pr_err("%-6s   %12llu   %12llu   %8lu   %10zu\n", "bitmap", alloc_time,
> +		free_time, capacity, memory);

I wonder if the numbers could be more useful if it is somehow normalized (e.g.
by how many iterations are performed?)? This number can be computed by adding
up the prefix of region_sizes until it cover all capacity.

Also, the capacity should probably be made part of the report header for all 3
ID allocators. Given this is randomized benchmark the single line, when taken
out of context, is not very meaningful anyway.

Something like this might be better (fake numbers below, you might also need
some fix point printing):

capacity = 1000000, regions = 100000
type       alloc (ns/region) free (ns/region) memory (B/capacity)
bitmap              1795.730           3.421                0.125
IDA                  465.556         339.314                0.134
maple                186.296         197.413                1.548

capacity = 100000, regions = 10000
bitmap               163.091           3.093                0.125
IDA                  614.478         335.459                0.142
maple                174.502         182.503                1.554

capacity = 10000, regions = 1000
bitmap                28.448           3.374                0.125
IDA                  418.978         333.641                0.187
maple                185.398         211.138                1.563

capacity = 1000, regions = 100
bitmap                22.530           6.100                0.128
IDA                  427.550         364.320                0.144
maple                197.280         234.740                1.552

Best,
Gary

> +
> +	return count;
> +}
> +
> +static size_t __init ida_memory(unsigned long nr_ids)
> +{
> +	unsigned long entries = DIV_ROUND_UP(nr_ids, IDA_BITMAP_BITS);
> +	unsigned long bitmaps = nr_ids / IDA_BITMAP_BITS;
> +	unsigned long nodes = 0;
> +
> +	if (nr_ids % IDA_BITMAP_BITS > BITS_PER_XA_VALUE)
> +		bitmaps++;
> +
> +	while (entries > 1) {
> +		entries = DIV_ROUND_UP(entries, XA_CHUNK_SIZE);
> +		nodes += entries;
> +	}
> +
> +	return sizeof(struct ida) + bitmaps * sizeof(struct ida_bitmap) +
> +		nodes * sizeof(struct xa_node);
> +}
> +
> +static unsigned long __init benchmark_ida(unsigned long capacity)
> +{
> +	struct ida ida = IDA_INIT(ida);
> +	unsigned long count, index, offset, nr_ids = 0;
> +	ktime_t alloc_time, free_time;
> +	int id = -ENOSPC;
> +
> +	alloc_time = ktime_get();
> +	for (count = 0; count < ARRAY_SIZE(region_sizes); count++) {
> +		for (offset = 0; offset < region_sizes[count]; offset++) {
> +			id = ida_alloc_max(&ida, capacity - 1, GFP_KERNEL);
> +			if (id < 0)
> +				break;
> +			if (!offset)
> +				region_indexes[count] = id;
> +		}
> +		if (id < 0) {
> +			while (offset--)
> +				ida_free(&ida, region_indexes[count] + offset);
> +			break;
> +		}
> +		WARN_ON(id != region_indexes[count] + region_sizes[count] - 1);
> +		nr_ids += region_sizes[count];
> +	}
> +	alloc_time = ktime_get() - alloc_time;
> +	WARN_ON(id != -ENOSPC);
> +
> +	index = count;
> +	free_time = ktime_get();
> +	while (index--)
> +		for (offset = 0; offset < region_sizes[index]; offset++)
> +			ida_free(&ida, region_indexes[index] + offset);
> +	free_time = ktime_get() - free_time;
> +	pr_err("%-6s   %12llu   %12llu   %8lu   %10zu\n", "IDA", alloc_time,
> +		free_time, capacity, ida_memory(nr_ids));
> +	ida_destroy(&ida);
> +
> +	return count;
> +}
> +