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Message-ID: <20210816154817.GM5469@sequoia>
Date: Mon, 16 Aug 2021 10:48:17 -0500
From: Tyler Hicks <tyhicks@...ux.microsoft.com>
To: Zhansaya Bagdauletkyzy <zhansayabagdaulet@...il.com>
Cc: akpm@...ux-foundation.org, shuah@...nel.org, linux-mm@...ck.org,
linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
pasha.tatashin@...een.com
Subject: Re: [PATCH v2 2/2] selftests: vm: add COW time test for KSM pages
On 2021-08-06 16:10:29, Zhansaya Bagdauletkyzy wrote:
> Since merged pages are copied every time they need to be modified,
> the write access time is different between shared and non-shared pages.
> Add ksm_cow_time() function which evaluates latency of these COW
> breaks. First, 4000 pages are allocated and the time, required to modify
> 1 byte in every other page, is measured. After this, the pages are
> merged into 2000 pairs and in each pair, 1 page is modified (i.e. they
> are decoupled) to detect COW breaks. The time needed to break COW of
> merged pages is then compared with performance of non-shared pages.
>
> The test is run as follows: ./ksm_tests -C
> The output:
> Total size: 15 MiB
>
> Not merged pages:
> Total time: 0.001903112 s
> Average speed: 3678.186 MiB/s
>
> Merged pages:
> Total time: 0.006215680 s
> Average speed: 1126.184 MiB/s
>
> Signed-off-by: Zhansaya Bagdauletkyzy <zhansayabagdaulet@...il.com>
> ---
> v1 -> v2:
> As suggested by Pavel,
> - add baseline figures with non-shared pages
> - instead of having all pages merged together, create pairs of
> duplicated pages
>
> Pavel's review comments:
> https://lore.kernel.org/lkml/CA+CK2bDYZBBaU3pC369o01tCgydaJ6y91GZ0_MWONMMCajZOUw@mail.gmail.com/
>
> tools/testing/selftests/vm/ksm_tests.c | 84 +++++++++++++++++++++++++-
> 1 file changed, 81 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/vm/ksm_tests.c b/tools/testing/selftests/vm/ksm_tests.c
> index 432dfe615e50..382ee6ccd13a 100644
> --- a/tools/testing/selftests/vm/ksm_tests.c
> +++ b/tools/testing/selftests/vm/ksm_tests.c
> @@ -33,7 +33,8 @@ enum ksm_test_name {
> CHECK_KSM_UNMERGE,
> CHECK_KSM_ZERO_PAGE_MERGE,
> CHECK_KSM_NUMA_MERGE,
> - KSM_MERGE_TIME
> + KSM_MERGE_TIME,
> + KSM_COW_TIME
> };
>
> static int ksm_write_sysfs(const char *file_path, unsigned long val)
> @@ -98,7 +99,8 @@ static void print_help(void)
> " -U (page unmerging)\n"
> " -P evaluate merging time and speed.\n"
> " For this test, the size of duplicated memory area (in MiB)\n"
> - " must be provided using -s option\n\n");
> + " must be provided using -s option\n"
> + " -C evaluate the time required to break COW of merged pages.\n\n");
>
> printf(" -a: specify the access protections of pages.\n"
> " <prot> must be of the form [rwx].\n"
> @@ -455,6 +457,75 @@ static int ksm_merge_time(int mapping, int prot, int timeout, size_t map_size)
> return KSFT_FAIL;
> }
>
> +static int ksm_cow_time(int mapping, int prot, int timeout, size_t page_size)
> +{
> + void *map_ptr;
> + struct timespec start_time, end_time;
> + unsigned long cow_time_ns;
> + int page_count = 4000;
size_t is more correct.
> +
> + map_ptr = allocate_memory(NULL, prot, mapping, '*', page_size * page_count);
> + if (!map_ptr)
> + return KSFT_FAIL;
> +
> + if (clock_gettime(CLOCK_MONOTONIC_RAW, &start_time)) {
> + perror("clock_gettime");
> + return KSFT_FAIL;
> + }
> + for (size_t i = 0; i < page_count; i = i + 2)
^
I think it is best to use the correct test here even though you're
stepping by 2 and won't have any problems in practice:
i < (page_count - 1)
This should be changed in all for loops in this function.
> + memset(map_ptr + page_size * i, '-', 1);
> + if (clock_gettime(CLOCK_MONOTONIC_RAW, &end_time)) {
> + perror("clock_gettime");
> + return KSFT_FAIL;
> + }
> +
> + cow_time_ns = (end_time.tv_sec - start_time.tv_sec) * NSEC_PER_SEC +
> + (end_time.tv_nsec - start_time.tv_nsec);
> +
> + printf("Total size: %lu MiB\n\n", (page_size * page_count) / MB);
> + printf("Not merged pages:\n");
> + printf("Total time: %ld.%09ld s\n", cow_time_ns / NSEC_PER_SEC,
> + cow_time_ns % NSEC_PER_SEC);
> + printf("Average speed: %.3f MiB/s\n\n", ((page_size * (page_count / 2)) / MB) /
> + ((double)cow_time_ns / NSEC_PER_SEC));
> +
> + /* Create 2000 pairs of duplicated pages */
> + for (size_t i = 0; i < page_count; i = i + 2) {
> + memset(map_ptr + page_size * i, '+', i + 1);
> + memset(map_ptr + page_size * (i + 1), '+', i + 1);
> + }
Since i is being used as the third argument to memset(), this loop only
works if page_count is less than page_size. I could see someone quickly
bumping page_count higher and then unknowingly getting bad test results.
Lets gain a little headroom in page_count. We are creating page_count/2
pairs of duplicate pages so we can use these memset() calls:
memset(map_ptr + page_size * i, '+', i / 2 + 1);
memset(map_ptr + page_size * (i + 1), '+', i / 2 + 1);
Then add a short comment to the page_count declaration mentioning that
page_count must be < (page_size / 2).
Everything else looks good to me.
Tyler
> + if (ksm_merge_pages(map_ptr, page_size * page_count, start_time, timeout))
> + goto err_out;
> +
> + if (clock_gettime(CLOCK_MONOTONIC_RAW, &start_time)) {
> + perror("clock_gettime");
> + goto err_out;
> + }
> + for (size_t i = 0; i < page_count; i = i + 2)
> + memset(map_ptr + page_size * i, '-', 1);
> + if (clock_gettime(CLOCK_MONOTONIC_RAW, &end_time)) {
> + perror("clock_gettime");
> + goto err_out;
> + }
> +
> + cow_time_ns = (end_time.tv_sec - start_time.tv_sec) * NSEC_PER_SEC +
> + (end_time.tv_nsec - start_time.tv_nsec);
> +
> + printf("Merged pages:\n");
> + printf("Total time: %ld.%09ld s\n", cow_time_ns / NSEC_PER_SEC,
> + cow_time_ns % NSEC_PER_SEC);
> + printf("Average speed: %.3f MiB/s\n", ((page_size * (page_count / 2)) / MB) /
> + ((double)cow_time_ns / NSEC_PER_SEC));
> +
> + munmap(map_ptr, page_size * page_count);
> + return KSFT_PASS;
> +
> +err_out:
> + printf("Not OK\n");
> + munmap(map_ptr, page_size * page_count);
> + return KSFT_FAIL;
> +}
> +
> int main(int argc, char *argv[])
> {
> int ret, opt;
> @@ -468,7 +539,7 @@ int main(int argc, char *argv[])
> bool merge_across_nodes = KSM_MERGE_ACROSS_NODES_DEFAULT;
> long size_MB = 0;
>
> - while ((opt = getopt(argc, argv, "ha:p:l:z:m:s:MUZNP")) != -1) {
> + while ((opt = getopt(argc, argv, "ha:p:l:z:m:s:MUZNPC")) != -1) {
> switch (opt) {
> case 'a':
> prot = str_to_prot(optarg);
> @@ -522,6 +593,9 @@ int main(int argc, char *argv[])
> case 'P':
> test_name = KSM_MERGE_TIME;
> break;
> + case 'C':
> + test_name = KSM_COW_TIME;
> + break;
> default:
> return KSFT_FAIL;
> }
> @@ -571,6 +645,10 @@ int main(int argc, char *argv[])
> ret = ksm_merge_time(MAP_PRIVATE | MAP_ANONYMOUS, prot, ksm_scan_limit_sec,
> size_MB);
> break;
> + case KSM_COW_TIME:
> + ret = ksm_cow_time(MAP_PRIVATE | MAP_ANONYMOUS, prot, ksm_scan_limit_sec,
> + page_size);
> + break;
> }
>
> if (ksm_restore(&ksm_sysfs_old)) {
> --
> 2.25.1
>
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