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Date: Tue, 16 Jan 2018 18:09:12 +0300
From: Timofey Titovets <nefelim4ag@...il.com>
To: linux-mm@...ck.org
Cc: Linux Kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
Timofey Titovets <nefelim4ag@...il.com>,
leesioh <solee@...korea.ac.kr>,
Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH V6 2/2] ksm: replace jhash2 with faster hash
Gentle ping
2018-01-03 6:09 GMT+03:00 Timofey Titovets <nefelim4ag@...il.com>:
> 1. Pickup, Sioh Lee crc32 patch, after some long conversation
> 2. Merge with my work on xxhash
> 3. Add autoselect code to 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: xxh64 hash() 8770 MB/s
> ksm: xxh32 hash() 4529 MB/s
> ksm: jhash2 hash() 1569 MB/s
>
> As jhash2 always will be slower (for data size like PAGE_SIZE),
> just drop it from choice.
>
> Add function to autoselect hash algo on boot,
> based on hashing speed, like raid6 code does.
>
> Move init of zero_checksum from init, to first call of fasthash():
> 1. KSM Init run on early kernel init,
> run perf testing stuff on main kernel boot thread looks bad to me.
> 2. Crypto subsystem not avaliable at that early booting,
> so crc32c even, compiled in, not avaliable
> As crypto and ksm init, run at subsys_initcall() (4) kernel level of init,
> all possible consumers will run later at 5+ levels
>
> Output after first try of KSM to hash page:
> ksm: crc32c hash() 15218 MB/s
> ksm: xxhash hash() 8640 MB/s
> ksm: choice crc32c as hash function
>
> Thanks.
>
> Changes:
> v1 -> v2:
> - Move xxhash() to xxhash.h/c and separate patches
> v2 -> v3:
> - Move xxhash() xxhash.c -> xxhash.h
> - replace xxhash_t with 'unsigned long'
> - update kerneldoc above xxhash()
> v3 -> v4:
> - 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
> v4 -> v5:
> - Pickup missed xxhash patch
> - Update code with compile time choicen xxhash
> - Add more macros to make code more readable
> - As now that only possible use xxhash or crc32c,
> on crc32c allocation error, skip speed test and fallback to xxhash
> - For workaround too early init problem (crc32c not avaliable),
> move zero_checksum init to first call of fastcall()
> - Don't alloc page for hash testing, use arch zero pages for that
> v5 -> v6:
> - Use libcrc32c instead of CRYPTO API, mainly for
> code/Kconfig deps Simplification
> - Add crc32c_available():
> libcrc32c will BUG_ON on crc32c problems,
> so test crc32c avaliable by crc32c_available()
> - Simplify choice_fastest_hash()
> - Simplify fasthash()
> - struct rmap_item && stable_node have sizeof == 64 on x86_64,
> that makes them cache friendly. As we don't suffer from hash collisions,
> change hash type from unsigned long back to u32.
> - Fix kbuild robot warning, make all local functions static
>
> Signed-off-by: Timofey Titovets <nefelim4ag@...il.com>
> Signed-off-by: leesioh <solee@...korea.ac.kr>
> CC: Andrea Arcangeli <aarcange@...hat.com>
> CC: linux-mm@...ck.org
> CC: kvm@...r.kernel.org
> ---
> mm/Kconfig | 2 ++
> mm/ksm.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
> 2 files changed, 91 insertions(+), 4 deletions(-)
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 03ff7703d322..b60bee4bb07e 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -305,6 +305,8 @@ config MMU_NOTIFIER
> config KSM
> bool "Enable KSM for page merging"
> depends on MMU
> + select XXHASH
> + select LIBCRC32C
> 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..65665bce2dce 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 +40,13 @@
> #include <linux/numa.h>
>
> #include <asm/tlbflush.h>
> +
> +/* Support for xxhash and crc32c */
> +#include <crypto/hash.h>
> +#include <linux/crc32c.h>
> +#include <linux/xxhash.h>
> +#include <linux/sizes.h>
> +
> #include "internal.h"
>
> #ifdef CONFIG_NUMA
> @@ -284,6 +290,87 @@ static DEFINE_SPINLOCK(ksm_mmlist_lock);
> sizeof(struct __struct), __alignof__(struct __struct),\
> (__flags), NULL)
>
> +#define TIME_125MS (HZ >> 3)
> +#define PERF_TO_MBS(X) (X*PAGE_SIZE*(1 << 3)/(SZ_1M))
> +
> +#define HASH_NONE 0
> +#define HASH_CRC32C 1
> +#define HASH_XXHASH 2
> +
> +static int fastest_hash = HASH_NONE;
> +
> +static bool __init crc32c_available(void)
> +{
> + static struct shash_desc desc;
> +
> + desc.tfm = crypto_alloc_shash("crc32c", 0, 0);
> + desc.flags = 0;
> +
> + if (IS_ERR(desc.tfm)) {
> + pr_warn("ksm: alloc crc32c shash error %ld\n",
> + -PTR_ERR(desc.tfm));
> + return false;
> + }
> +
> + crypto_free_shash(desc.tfm);
> + return true;
> +}
> +
> +static void __init choice_fastest_hash(void)
> +{
> +
> + unsigned long je;
> + unsigned long perf_crc32c = 0;
> + unsigned long perf_xxhash = 0;
> +
> + fastest_hash = HASH_XXHASH;
> + if (!crc32c_available())
> + goto out;
> +
> + preempt_disable();
> + je = jiffies + TIME_125MS;
> + while (time_before(jiffies, je)) {
> + crc32c(0, ZERO_PAGE(0), PAGE_SIZE);
> + perf_crc32c++;
> + }
> + preempt_enable();
> +
> + preempt_disable();
> + je = jiffies + TIME_125MS;
> + while (time_before(jiffies, je)) {
> + xxhash(ZERO_PAGE(0), PAGE_SIZE, 0);
> + perf_xxhash++;
> + }
> + preempt_enable();
> +
> + pr_info("ksm: crc32c hash() %5ld MB/s\n", PERF_TO_MBS(perf_crc32c));
> + pr_info("ksm: xxhash hash() %5ld MB/s\n", PERF_TO_MBS(perf_xxhash));
> +
> + if (perf_crc32c > perf_xxhash)
> + fastest_hash = HASH_CRC32C;
> +out:
> + if (fastest_hash == HASH_CRC32C)
> + pr_info("ksm: choice crc32c as hash function\n");
> + else
> + pr_info("ksm: choice xxhash as hash function\n");
> +}
> +
> +static u32 fasthash(const void *input, size_t length)
> +{
> +again:
> + switch (fastest_hash) {
> + case HASH_CRC32C:
> + return crc32c(0, input, length);
> + case HASH_XXHASH:
> + return xxhash(input, length, 0);
> + default:
> + choice_fastest_hash();
> + /* The correct value depends on page size and endianness */
> + zero_checksum = fasthash(ZERO_PAGE(0), PAGE_SIZE);
> + goto again;
> + }
> +}
> +
> static int __init ksm_slab_init(void)
> {
> rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
> @@ -986,7 +1073,7 @@ static u32 calc_checksum(struct page *page)
> {
> u32 checksum;
> void *addr = kmap_atomic(page);
> - checksum = jhash2(addr, PAGE_SIZE / 4, 17);
> + checksum = fasthash(addr, PAGE_SIZE);
> kunmap_atomic(addr);
> return checksum;
> }
> @@ -3068,8 +3155,6 @@ 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
--
Have a nice day,
Timofey.
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