diff mbox

[v2,19/27] target-i386: remove helper_lock()

Message ID 1467392693-22715-20-git-send-email-rth@twiddle.net
State New
Headers show

Commit Message

Richard Henderson July 1, 2016, 5:04 p.m. UTC 
From: "Emilio G. Cota" <cota@braap.org>

It's been superseded by the atomic helpers.

The use of the atomic helpers provides a significant performance and scalability
improvement. Below is the result of running the atomic_add-test microbenchmark with:
 $ x86_64-linux-user/qemu-x86_64 tests/atomic_add-bench -o 5000000 -r $r -n $n
, where $n is the number of threads and $r is the allowed range for the additions.

The scenarios measured are:
- atomic: implements x86' ADDL with the atomic_add helper (i.e. this patchset)
- cmpxchg: implement x86' ADDL with a TCG loop using the cmpxchg helper
- master: before this patchset

Results sorted in ascending range, i.e. descending degree of contention.
Y axis is Throughput in Mops/s. Tests are run on an AMD machine with 64
Opteron 6376 cores.

                atomic_add-bench: 5000000 ops/thread, [0,1] range

  25 ++---------+----------+---------+----------+----------+----------+---++
     + atomic +-E--+       +         +          +          +          +    |
     |cmpxchg +-H--+                                                       |
  20 +Emaster +-N--+                                                      ++
     ||                                                                    |
     |++                                                                   |
     ||                                                                    |
  15 +++                                                                  ++
     |N|                                                                   |
     |+|                                                                   |
  10 ++|                                                                  ++
     |+|+                                                                  |
     | |    -+E+------        +++  ---+E+------+E+------+E+-----+E+------+E|
     |+E+E+- +++     +E+------+E+--                                        |
   5 ++|+                                                                 ++
     |+N+H+---                                 +++                         |
     ++++N+--+H++----+++   +  +++  --++H+------+H+------+H++----+H+---+--- |
   0 ++---------+-----H----+---H-----+----------+----------+----------+---H+
     0          10         20        30         40         50         60
                                Number of threads

                atomic_add-bench: 5000000 ops/thread, [0,2] range

  25 ++---------+----------+---------+----------+----------+----------+---++
     ++atomic +-E--+       +         +          +          +          +    |
     |cmpxchg +-H--+                                                       |
  20 ++master +-N--+                                                      ++
     |E|                                                                   |
     |++                                                                   |
     ||E                                                                   |
  15 ++|                                                                  ++
     |N||                                                                  |
     |+||                                   ---+E+------+E+-----+E+------+E|
  10 ++| |        ---+E+------+E+-----+E+---                    +++      +++
     ||H+E+--+E+--                                                         |
     |+++++                                                                |
     | ||                                                                  |
   5 ++|+H+--                                  +++                        ++
     |+N+    -                              ---+H+------+H+------          |
     +  +N+--+H++----+H+---+--+H+----++H+---    +          +    +H+---+--+H|
   0 ++---------+----------+---------+----------+----------+----------+---++
     0          10         20        30         40         50         60
                                Number of threads

                atomic_add-bench: 5000000 ops/thread, [0,8] range

  40 ++---------+----------+---------+----------+----------+----------+---++
     ++atomic +-E--+       +         +          +          +          +    |
  35 +cmpxchg +-H--+                                                      ++
     | master +-N--+               ---+E+------+E+------+E+-----+E+------+E|
  30 ++|                   ---+E+--   +++                                 ++
     | |            -+E+---                                                |
  25 ++E        ---- +++                                                  ++
     |+++++ -+E+                                                           |
  20 +E+ E-- +++                                                          ++
     |H|+++                                                                |
     |+|                                       +H+-------                  |
  15 ++H+                                   ---+++      +H+------         ++
     |N++H+--                         +++---                    +H+------++|
  10 ++ +++  -       +++           ---+H+                       +++      +H+
     | |     +H+-----+H+------+H+--                                        |
   5 ++|                      +++                                         ++
     ++N+N+--+N++          +         +          +          +          +    |
   0 ++---------+----------+---------+----------+----------+----------+---++
     0          10         20        30         40         50         60
                                Number of threads

               atomic_add-bench: 5000000 ops/thread, [0,128] range

  160 ++---------+---------+----------+---------+----------+----------+---++
      + atomic +-E--+      +          +         +          +          +    |
  140 +cmpxchg +-H--+                          +++      +++               ++
      | master +-N--+                           E--------E------+E+------++|
  120 ++                                      --|        |      +++       E+
      |                                     -- +++      +++              ++|
  100 ++                                   -                              ++
      |                                +++-                     +++      ++|
   80 ++                              -+E+    -+H+------+H+------H--------++
      |                           ----    ----                  +++       H|
      |            ---+E+-----+E+-  ---+H+                               ++|
   60 ++     +E+---   +++  ---+H+---                                      ++
      |    --+++   ---+H+--                                                |
   40 ++ +E+-+H+---                                                       ++
      |  +H+                                                               |
   20 +EE+                                                                ++
      +N+        +         +          +         +          +          +    |
    0 ++N-N---N--+---------+----------+---------+----------+----------+---++
      0          10        20         30        40         50         60
                                Number of threads

              atomic_add-bench: 5000000 ops/thread, [0,1024] range

  350 ++---------+---------+----------+---------+----------+----------+---++
      + atomic +-E--+      +          +         +          +          +    |
  300 +cmpxchg +-H--+                                                    +++
      | master +-N--+                                           +++       ||
      |                                                 +++      |    ----E|
  250 ++                                                 |   ----E----    ++
      |                                              ----E---    |    ---+H|
  200 ++                                      -+E+---   +++  ---+H+---    ++
      |                                   ----         -+H+--              |
      |                                +E+     +++ ---- +++                |
  150 ++                            ---+++  ---+H+-                       ++
      |                          ---  -+H+--                               |
  100 ++                   ---+E+ ---- +++                                ++
      |      +++   ---+E+-----+H+-                                         |
      |     -+E+------+H+--                                                |
   50 ++ +E+                                                              ++
      +EE+       +         +          +         +          +          +    |
    0 ++N-N---N--+---------+----------+---------+----------+----------+---++
      0          10        20         30        40         50         60
                                Number of threads

  hi-res: http://imgur.com/a/fMRmq

For master I stopped measuring master after 8 threads, because there is little
point in measuring the well-known performance collapse of a contended lock.

Note that using atomic helpers instead of cmpxchg is not only more performant
(when the host implements natively the same atomics, as is in this case); it also
simplifies the necessary TCG/helper code of the implementation. Compare
the difference between the atomic and cmpxchg implementations:

     case OP_ADDL:
         if (s1->prefix & PREFIX_LOCK) {
-            gen_atomic_add_fetch(cpu_T0, cpu_A0, cpu_T1, ot);
+            TCGv t0, t1, t2, a0;
+            TCGLabel *retry;
+
+            t0 = tcg_temp_local_new();
+            t1 = tcg_temp_local_new();
+            t2 = tcg_temp_local_new();
+            a0 = tcg_temp_local_new();
+            retry = gen_new_label();
+
+            tcg_gen_mov_tl(a0, cpu_A0);
+            tcg_gen_mov_tl(t1, cpu_T1);
+            gen_set_label(retry);
+            gen_op_ld_v(s1, ot, cpu_T0, a0);
+            tcg_gen_add_tl(t2, cpu_T0, t1);
+            gen_cmpxchg(t0, a0, cpu_T0, t2, ot);
+            tcg_gen_brcond_tl(TCG_COND_NE, t0, cpu_T0, retry);
+
+            tcg_gen_mov_tl(cpu_T0, t2);
+            tcg_gen_mov_tl(cpu_T1, t1);
+
+            tcg_temp_free(t0);
+            tcg_temp_free(t1);
+            tcg_temp_free(t2);
+            tcg_temp_free(a0);
         } else {

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1467054136-10430-21-git-send-email-cota@braap.org>
---
 target-i386/helper.h     |  2 --
 target-i386/mem_helper.c | 33 ---------------------------------
 target-i386/translate.c  | 15 ---------------
 3 files changed, 50 deletions(-)
diff mbox

Patch

diff --git a/target-i386/helper.h b/target-i386/helper.h
index 1320edc..d02357c 100644
--- a/target-i386/helper.h
+++ b/target-i386/helper.h
@@ -1,8 +1,6 @@ 
 DEF_HELPER_FLAGS_4(cc_compute_all, TCG_CALL_NO_RWG_SE, tl, tl, tl, tl, int)
 DEF_HELPER_FLAGS_4(cc_compute_c, TCG_CALL_NO_RWG_SE, tl, tl, tl, tl, int)
 
-DEF_HELPER_0(lock, void)
-DEF_HELPER_0(unlock, void)
 DEF_HELPER_3(write_eflags, void, env, tl, i32)
 DEF_HELPER_1(read_eflags, tl, env)
 DEF_HELPER_2(divb_AL, void, env, tl)
diff --git a/target-i386/mem_helper.c b/target-i386/mem_helper.c
index 5c0558f..8649115 100644
--- a/target-i386/mem_helper.c
+++ b/target-i386/mem_helper.c
@@ -25,39 +25,6 @@ 
 #include "qemu/int128.h"
 #include "tcg.h"
 
-/* broken thread support */
-
-#if defined(CONFIG_USER_ONLY)
-QemuMutex global_cpu_lock;
-
-void helper_lock(void)
-{
-    qemu_mutex_lock(&global_cpu_lock);
-}
-
-void helper_unlock(void)
-{
-    qemu_mutex_unlock(&global_cpu_lock);
-}
-
-void helper_lock_init(void)
-{
-    qemu_mutex_init(&global_cpu_lock);
-}
-#else
-void helper_lock(void)
-{
-}
-
-void helper_unlock(void)
-{
-}
-
-void helper_lock_init(void)
-{
-}
-#endif
-
 void helper_cmpxchg8b(CPUX86State *env, target_ulong a0)
 {
     uintptr_t ra = GETPC();
diff --git a/target-i386/translate.c b/target-i386/translate.c
index 525c445..9468cd5 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -4537,10 +4537,6 @@  static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
     s->aflag = aflag;
     s->dflag = dflag;
 
-    /* lock generation */
-    if (prefixes & PREFIX_LOCK)
-        gen_helper_lock();
-
     /* now check op code */
  reswitch:
     switch(b) {
@@ -8195,20 +8191,11 @@  static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
     default:
         goto unknown_op;
     }
-    /* lock generation */
-    if (s->prefix & PREFIX_LOCK)
-        gen_helper_unlock();
     return s->pc;
  illegal_op:
-    if (s->prefix & PREFIX_LOCK)
-        gen_helper_unlock();
-    /* XXX: ensure that no lock was generated */
     gen_illegal_opcode(s);
     return s->pc;
  unknown_op:
-    if (s->prefix & PREFIX_LOCK)
-        gen_helper_unlock();
-    /* XXX: ensure that no lock was generated */
     gen_unknown_opcode(env, s);
     return s->pc;
 }
@@ -8300,8 +8287,6 @@  void tcg_x86_init(void)
                                      offsetof(CPUX86State, bnd_regs[i].ub),
                                      bnd_regu_names[i]);
     }
-
-    helper_lock_init();
 }
 
 /* generate intermediate code for basic block 'tb'.  */