tb hash: hash phys_pc, pc, and flags with xxhash

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:

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: Sergey Fedorov <sergey.fedorov@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1465412133-3029-8-git-send-email-cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/cpu-exec.c b/cpu-exec.c
index f7c642f4a9..b9e294c0e6 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -232,13 +232,13 @@ 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];