Re: [PATCH V6 2/2] ksm: replace jhash2 with faster hash

[Timofey Titovets]

Gentle ping

2018-01-03 6:09 GMT+03:00 Timofey Titovets <nefelim4ag@xxxxxxxxx>:
> 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@xxxxxxxxx>
> Signed-off-by: leesioh <solee@xxxxxxxxxxxxxx>
> CC: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> CC: linux-mm@xxxxxxxxx
> CC: kvm@xxxxxxxxxxxxxxx
> ---
>  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.