Re: [Qemu-devel] [PATCH v5 09/18] tb hash: hash phys_pc, pc, and flags with xxhash

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 14/05/16 06:34, Emilio G. Cota wrote:
> For some workloads such as arm bootup, tb_phys_hash is performance-critical.
> The is due to the high frequency of accesses to the hash table, originated
> by (frequent) TLB flushes that wipe out the cpu-private tb_jmp_cache's.
> More info:
>   https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg05098.html
>
> To dig further into this I modified an arm image booting debian jessie to
> immediately shut down after boot. Analysis revealed that quite a bit of time
> is unnecessarily spent in tb_phys_hash: the cause is poor hashing that
> results in very uneven loading of chains in the hash table's buckets;
> the longest observed chain had ~550 elements.
>
> The appended addresses this with two changes:

Does "the appended" means "this patch"? Sorry, I've just never seen such
expression before...

>
> 1) Use xxhash as the hash table's hash function. xxhash is a fast,
>    high-quality hashing function.
>
> 2) Feed the hashing function with not just tb_phys, but also pc and flags.
>
> This improves performance over using just tb_phys for hashing, since that
> resulted in some hash buckets having many TB's, while others getting very few;
> with these changes, the longest observed chain on a single hash bucket is
> brought down from ~550 to ~40.
>
> Tests show that the other element checked for in tb_find_physical,
> cs_base, is always a match when tb_phys+pc+flags are a match,
> so hashing cs_base is wasteful. It could be that this is an ARM-only
> thing, though. UPDATE:
> On Tue, Apr 05, 2016 at 08:41:43 -0700, Richard Henderson wrote:
>> The cs_base field is only used by i386 (in 16-bit modes), and sparc (for a TB
>> consisting of only a delay slot).
>> It may well still turn out to be reasonable to ignore cs_base for hashing.
> BTW, after this change the hash table should not be called "tb_hash_phys"
> anymore; this is addressed later in this series.
>
> This change gives consistent bootup time improvements. I tested two
> host machines:
> - Intel Xeon E5-2690: 11.6% less time
> - Intel i7-4790K: 19.2% less time
>
> Increasing the number of hash buckets yields further improvements. However,
> using a larger, fixed number of buckets can degrade performance for other
> workloads that do not translate as many blocks (600K+ for debian-jessie arm
> bootup). This is dealt with later in this series.
>
> Reviewed-by: Richard Henderson <rth@twiddle.net>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
> Signed-off-by: Emilio G. Cota <cota@braap.org>
> ---
>  cpu-exec.c             |  4 ++--
>  include/exec/tb-hash.h |  8 ++++++--
>  translate-all.c        | 10 +++++-----
>  3 files changed, 13 insertions(+), 9 deletions(-)
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 14df1aa..1735032 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -231,13 +231,13 @@ static TranslationBlock *tb_find_physical(CPUState *cpu,
>  {
>      CPUArchState *env = (CPUArchState *)cpu->env_ptr;
>      TranslationBlock *tb, **tb_hash_head, **ptb1;
> -    unsigned int h;
> +    uint32_t h;
>      tb_page_addr_t phys_pc, phys_page1;
>  
>      /* find translated block using physical mappings */
>      phys_pc = get_page_addr_code(env, pc);
>      phys_page1 = phys_pc & TARGET_PAGE_MASK;
> -    h = tb_phys_hash_func(phys_pc);
> +    h = tb_hash_func(phys_pc, pc, flags);
>  
>      /* Start at head of the hash entry */
>      ptb1 = tb_hash_head = &tcg_ctx.tb_ctx.tb_phys_hash[h];
> diff --git a/include/exec/tb-hash.h b/include/exec/tb-hash.h
> index 0f4e8a0..4b9635a 100644
> --- a/include/exec/tb-hash.h
> +++ b/include/exec/tb-hash.h
> @@ -20,6 +20,9 @@
>  #ifndef EXEC_TB_HASH
>  #define EXEC_TB_HASH
>  
> +#include "exec/exec-all.h"
> +#include "exec/tb-hash-xx.h"
> +
>  /* Only the bottom TB_JMP_PAGE_BITS of the jump cache hash bits vary for
>     addresses on the same page.  The top bits are the same.  This allows
>     TLB invalidation to quickly clear a subset of the hash table.  */
> @@ -43,9 +46,10 @@ static inline unsigned int tb_jmp_cache_hash_func(target_ulong pc)
>             | (tmp & TB_JMP_ADDR_MASK));
>  }
>  
> -static inline unsigned int tb_phys_hash_func(tb_page_addr_t pc)
> +static inline
> +uint32_t tb_hash_func(tb_page_addr_t phys_pc, target_ulong pc, int flags)

Nitpicking: now 'flags' is of uint32_t type.

>  {
> -    return (pc >> 2) & (CODE_GEN_PHYS_HASH_SIZE - 1);
> +    return tb_hash_func5(phys_pc, pc, flags) & (CODE_GEN_PHYS_HASH_SIZE - 1);
>  }
>  
>  #endif
> diff --git a/translate-all.c b/translate-all.c
> index b54f472..c48fccb 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -991,12 +991,12 @@ void tb_phys_invalidate(TranslationBlock *tb, tb_page_addr_t page_addr)
>  {
>      CPUState *cpu;
>      PageDesc *p;
> -    unsigned int h;
> +    uint32_t h;
>      tb_page_addr_t phys_pc;
>  
>      /* remove the TB from the hash list */
>      phys_pc = tb->page_addr[0] + (tb->pc & ~TARGET_PAGE_MASK);
> -    h = tb_phys_hash_func(phys_pc);
> +    h = tb_hash_func(phys_pc, tb->pc, tb->flags);
>      tb_hash_remove(&tcg_ctx.tb_ctx.tb_phys_hash[h], tb);
>  
>      /* remove the TB from the page list */
> @@ -1126,11 +1126,11 @@ static inline void tb_alloc_page(TranslationBlock *tb,
>  static void tb_link_page(TranslationBlock *tb, tb_page_addr_t phys_pc,
>                           tb_page_addr_t phys_page2)
>  {
> -    unsigned int h;
> +    uint32_t h;
>      TranslationBlock **ptb;
>  
> -    /* add in the physical hash table */
> -    h = tb_phys_hash_func(phys_pc);
> +    /* add in the hash table */
> +    h = tb_hash_func(phys_pc, tb->pc, tb->flags);
>      ptb = &tcg_ctx.tb_ctx.tb_phys_hash[h];
>      tb->phys_hash_next = *ptb;
>      *ptb = tb;

Kind regards,
Sergey