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SubjectRe: [PATCH v2] ksm: replace jhash2 with faster hash
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hello

First, thanks for organizing all the experiments.

and i'm sending you the results of experiments

Test platform: openstack cloud platform (NEWTON version)
Experiment node: openstack based cloud compute node (CPU: xeon E5-2620 v3, memory 64gb)
VM: (2 VCPU, RAM 4GB, DISK 20GB) * 4
Linux kernel: 4.14 (latest version)
KSM setup - sleep_millisecs: 200ms, pages_to_scan: 200

Experiment process
Firstly, we turn off KSM and launch 4 VMs.
Then we turn on the KSM and measure the checksum computation time until full_scans become two.

The experimental results (the experimental value is the average of the measured values)
crc32c_intel: 1084.10ns
crc32c (no hardware acceleration): 7012.51ns
xxhash32: 2227.75ns
xxhash64: 1413.16ns
jhash2: 5128.30ns

In summary, the result shows that crc32c_intel has advantages over all of the hash function used in the experiment. (decreased by 84.54% compared to crc32c, 78.86% compared to jhash2, 51.33% xxhash32, 23.28% compared to xxhash64)

the results are similar to those of Timofey.

anyway, i saw the problem of Timofey and i had the same situation before.

the solution is to call crc32c using crce32c library instead of shash alloc (e.g. checksum = crc32c(0,addr,PAGE_SIZE);)

and change code from subsys_initcall(ksm_init)  to  late_initcall(ksm_init).

I have solved kernel problem using this method so this will be helpful.

please tell me if other problems exists.

thanks.

-sioh lee-


2017-12-31 오전 6:27에 Timofey Titovets 이(가) 쓴 글:
> *FACEPALM*,
> Sorry, just forgot about numbering of old jhash2 -> xxhash conversion
> Also pickup patch for xxhash - arch dependent xxhash() function that will use
> fastest algo for current arch.
>
> So next will be v5, as that must be v4.
>
> Thanks.
>
> 2017-12-29 12:52 GMT+03:00 Timofey Titovets <nefelim4ag@gmail.com>:
>> Pickup, Sioh Lee crc32 patch, after some long conversation
>> and hassles, merge with my work on xxhash, add
>> choice fastest hash helper.
>>
>> Base idea are same, replace jhash2 with something faster.
>>
>> Perf numbers:
>> Intel(R) Xeon(R) CPU E5-2420 v2 @ 2.20GHz
>> ksm: crc32c hash() 12081 MB/s
>> ksm: jhash2 hash() 1569 MB/s
>> ksm: xxh64 hash() 8770 MB/s
>> ksm: xxh32 hash() 4529 MB/s
>>
>> As jhash2 always will be slower, just drop it from choice.
>>
>> Add function to autoselect hash algo on boot, based on speed,
>> like raid6 code does.
>>
>> Move init of zero_hash from init, to start of ksm thread,
>> as ksm init run on early kernel init, run perf testing stuff on
>> main kernel thread looks bad to me.
>>
>> One problem exists with that patch,
>> ksm init run too early, and crc32c module, even compiled in
>> can't be found, so i see:
>> - ksm: alloc crc32c shash error 2 in dmesg.
>>
>> I give up on that, so ideas welcomed.
>>
>> Only idea that i have, are to avoid early init by moving
>> zero_checksum to sysfs_store parm,
>> i.e. that's default to false, and that will work, i think.
>>
>> Thanks.
>>
>> Changes:
>> v1 -> v2:
>> - Merge xxhash/crc32 patches
>> - Replace crc32 with crc32c (crc32 have same as jhash2 speed)
>> - Add auto speed test and auto choice of fastest hash function
>>
>> Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
>> Signed-off-by: leesioh <solee@os.korea.ac.kr>
>> CC: Andrea Arcangeli <aarcange@redhat.com>
>> CC: linux-mm@kvack.org
>> CC: kvm@vger.kernel.org
>> ---
>> mm/Kconfig | 4 ++
>> mm/ksm.c | 133 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
>> 2 files changed, 128 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index 03ff7703d322..d4fb147d4a22 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -305,6 +305,10 @@ config MMU_NOTIFIER
>> config KSM
>> bool "Enable KSM for page merging"
>> depends on MMU
>> + select XXHASH
>> + select CRYPTO
>> + select CRYPTO_HASH
>> + select CONFIG_CRYPTO_CRC32C
>> help
>> Enable Kernel Samepage Merging: KSM periodically scans those areas
>> of an application's address space that an app has advised may be
>> diff --git a/mm/ksm.c b/mm/ksm.c
>> index be8f4576f842..fd5c9d0f7bc2 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -25,7 +25,6 @@
>> #include <linux/pagemap.h>
>> #include <linux/rmap.h>
>> #include <linux/spinlock.h>
>> -#include <linux/jhash.h>
>> #include <linux/delay.h>
>> #include <linux/kthread.h>
>> #include <linux/wait.h>
>> @@ -41,6 +40,12 @@
>> #include <linux/numa.h>
>>
>> #include <asm/tlbflush.h>
>> +
>> +/* Support for xxhash and crc32c */
>> +#include <linux/crypto.h>
>> +#include <crypto/hash.h>
>> +#include <linux/xxhash.h>
>> +
>> #include "internal.h"
>>
>> #ifdef CONFIG_NUMA
>> @@ -186,7 +191,7 @@ struct rmap_item {
>> };
>> struct mm_struct *mm;
>> unsigned long address; /* + low bits used for flags below */
>> - unsigned int oldchecksum; /* when unstable */
>> + unsigned long oldchecksum; /* when unstable */
>> union {
>> struct rb_node node; /* when node of unstable tree */
>> struct { /* when listed from stable tree */
>> @@ -255,7 +260,7 @@ static unsigned int ksm_thread_pages_to_scan = 100;
>> static unsigned int ksm_thread_sleep_millisecs = 20;
>>
>> /* Checksum of an empty (zeroed) page */
>> -static unsigned int zero_checksum __read_mostly;
>> +static unsigned long zero_checksum __read_mostly;
>>
>> /* Whether to merge empty (zeroed) pages with actual zero pages */
>> static bool ksm_use_zero_pages __read_mostly;
>> @@ -284,6 +289,115 @@ static DEFINE_SPINLOCK(ksm_mmlist_lock);
>> sizeof(struct __struct), __alignof__(struct __struct),\
>> (__flags), NULL)
>>
>> +#define CRC32C_HASH 1
>> +#define XXH32_HASH 2
>> +#define XXH64_HASH 3
>> +
>> +const static char *hash_func_names[] = { "", "crc32c", "xxh32", "xxh64" };
>> +
>> +static struct shash_desc desc;
>> +static struct crypto_shash *tfm;
>> +static uint8_t fastest_hash = 0;
>> +
>> +static void __init choice_fastest_hash(void)
>> +{
>> + void *page = kmalloc(PAGE_SIZE, GFP_KERNEL);
>> + unsigned long checksum, perf, js, je;
>> + unsigned long best_perf = 0;
>> +
>> + tfm = crypto_alloc_shash(hash_func_names[CRC32C_HASH],
>> + CRYPTO_ALG_TYPE_SHASH, 0);
>> +
>> + if (IS_ERR(tfm)) {
>> + pr_warn("ksm: alloc %s shash error %ld\n",
>> + hash_func_names[CRC32C_HASH], -PTR_ERR(tfm));
>> + } else {
>> + desc.tfm = tfm;
>> + desc.flags = 0;
>> +
>> + perf = 0;
>> + preempt_disable();
>> + js = jiffies;
>> + je = js + (HZ >> 3);
>> + while (time_before(jiffies, je)) {
>> + crypto_shash_digest(&desc, page, PAGE_SIZE,
>> + (u8 *)&checksum);
>> + perf++;
>> + }
>> + preempt_enable();
>> + if (best_perf < perf) {
>> + best_perf = perf;
>> + fastest_hash = CRC32C_HASH;
>> + }
>> + pr_info("ksm: %-8s hash() %5ld MB/s\n",
>> + hash_func_names[CRC32C_HASH], perf*PAGE_SIZE >> 17);
>> + }
>> +
>> + perf = 0;
>> + preempt_disable();
>> + js = jiffies;
>> + je = js + (HZ >> 3);
>> + while (time_before(jiffies, je)) {
>> + checksum = xxh32(page, PAGE_SIZE, 0);
>> + perf++;
>> + }
>> + preempt_enable();
>> + if (best_perf < perf) {
>> + best_perf = perf;
>> + fastest_hash = XXH32_HASH;
>> + }
>> + pr_info("ksm: %-8s hash() %5ld MB/s\n",
>> + hash_func_names[XXH32_HASH], perf*PAGE_SIZE >> 17);
>> +
>> + perf = 0;
>> + preempt_disable();
>> + js = jiffies;
>> + je = js + (HZ >> 3);
>> + while (time_before(jiffies, je)) {
>> + checksum = xxh64(page, PAGE_SIZE, 0);
>> + perf++;
>> + }
>> + preempt_enable();
>> + if (best_perf < perf) {
>> + best_perf = perf;
>> + fastest_hash = XXH64_HASH;
>> + }
>> + pr_info("ksm: %-8s hash() %5ld MB/s\n",
>> + hash_func_names[XXH64_HASH], perf*PAGE_SIZE >> 17);
>> +
>> + if (!IS_ERR(tfm) && fastest_hash != CRC32C_HASH)
>> + crypto_free_shash(tfm);
>> +
>> + pr_info("ksm: choise %s as hash function\n",
>> + hash_func_names[fastest_hash]);
>> +
>> + kfree(page);
>> +}
>> +
>> +unsigned long fasthash(const void *input, size_t length)
>> +{
>> + unsigned long checksum = 0;
>> +
>> + switch (fastest_hash) {
>> + case 0:
>> + choice_fastest_hash();
>> + checksum = fasthash(input, length);
>> + break;
>> + case CRC32C_HASH:
>> + crypto_shash_digest(&desc, input, length,
>> + (u8 *)&checksum);
>> + break;
>> + case XXH32_HASH:
>> + checksum = xxh32(input, length, 0);
>> + break;
>> + case XXH64_HASH:
>> + checksum = xxh64(input, length, 0);
>> + break;
>> + }
>> +
>> + return checksum;
>> +}
>> +
>> static int __init ksm_slab_init(void)
>> {
>> rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
>> @@ -982,11 +1096,11 @@ static int unmerge_and_remove_all_rmap_items(void)
>> }
>> #endif /* CONFIG_SYSFS */
>>
>> -static u32 calc_checksum(struct page *page)
>> +static unsigned long calc_checksum(struct page *page)
>> {
>> - u32 checksum;
>> + unsigned long checksum;
>> void *addr = kmap_atomic(page);
>> - checksum = jhash2(addr, PAGE_SIZE / 4, 17);
>> + checksum = fasthash(addr, PAGE_SIZE);
>> kunmap_atomic(addr);
>> return checksum;
>> }
>> @@ -2006,7 +2120,7 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
>> struct page *tree_page = NULL;
>> struct stable_node *stable_node;
>> struct page *kpage;
>> - unsigned int checksum;
>> + unsigned long checksum;
>> int err;
>> bool max_page_sharing_bypass = false;
>>
>> @@ -2336,6 +2450,9 @@ static int ksm_scan_thread(void *nothing)
>> set_freezable();
>> set_user_nice(current, 5);
>>
>> + /* The correct value depends on page size and endianness */
>> + zero_checksum = calc_checksum(ZERO_PAGE(0));
>> +
>> while (!kthread_should_stop()) {
>> mutex_lock(&ksm_thread_mutex);
>> wait_while_offlining();
>> @@ -3068,8 +3185,6 @@ static int __init ksm_init(void)
>> struct task_struct *ksm_thread;
>> int err;
>>
>> - /* The correct value depends on page size and endianness */
>> - zero_checksum = calc_checksum(ZERO_PAGE(0));
>> /* Default to false for backwards compatibility */
>> ksm_use_zero_pages = false;
>>
>> --
>> 2.15.1
>
>

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