[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Qemu-devel] [RFC 6/6] cputlb: dynamically resize TLBs based on use rate
|
From: |
Emilio G. Cota |
|
Subject: |
[Qemu-devel] [RFC 6/6] cputlb: dynamically resize TLBs based on use rate |
|
Date: |
Sat, 6 Oct 2018 17:45:08 -0400 |
Perform the resizing only on flushes, otherwise we'd
have to take a perf hit by either rehashing the array
or unnecessarily flushing it.
We grow the array aggressively, and reduce the size more
slowly. This accommodates mixed workloads, where some
processes might be memory-heavy while others are not.
As the following experiments show, this a net perf gain,
particularly for memory-heavy workloads. Experiments
are run on an Intel i7-6700K CPU @ 4.00GHz.
1. System boot + shudown, debian aarch64:
- Before (tb-lock-v3):
Performance counter stats for 'taskset -c 0 ../img/aarch64/die.sh' (10 runs):
7469.363393 task-clock (msec) # 0.998 CPUs utilized
( +- 0.07% )
31,507,707,190 cycles # 4.218 GHz
( +- 0.07% )
57,101,577,452 instructions # 1.81 insns per cycle
( +- 0.08% )
10,265,531,804 branches # 1374.352 M/sec
( +- 0.07% )
173,020,681 branch-misses # 1.69% of all branches
( +- 0.10% )
7.483359063 seconds time elapsed
( +- 0.08% )
- After:
Performance counter stats for 'taskset -c 0 ../img/aarch64/die.sh' (10 runs):
7185.036730 task-clock (msec) # 0.999 CPUs utilized
( +- 0.11% )
30,303,501,143 cycles # 4.218 GHz
( +- 0.11% )
54,198,386,487 instructions # 1.79 insns per cycle
( +- 0.08% )
9,726,518,945 branches # 1353.719 M/sec
( +- 0.08% )
167,082,307 branch-misses # 1.72% of all branches
( +- 0.08% )
7.195597842 seconds time elapsed
( +- 0.11% )
That is, a 3.8% improvement.
2. System boot + shutdown, ubuntu 18.04 x86_64:
- Before (tb-lock-v3):
Performance counter stats for 'taskset -c 0 ../img/x86_64/ubuntu-die.sh
-nographic' (2 runs):
49971.036482 task-clock (msec) # 0.999 CPUs utilized
( +- 1.62% )
210,766,077,140 cycles # 4.218 GHz
( +- 1.63% )
428,829,830,790 instructions # 2.03 insns per cycle
( +- 0.75% )
77,313,384,038 branches # 1547.164 M/sec
( +- 0.54% )
835,610,706 branch-misses # 1.08% of all branches
( +- 2.97% )
50.003855102 seconds time elapsed
( +- 1.61% )
- After:
Performance counter stats for 'taskset -c 0 ../img/x86_64/ubuntu-die.sh
-nographic' (2 runs):
50118.124477 task-clock (msec) # 0.999 CPUs utilized
( +- 4.30% )
132,396 context-switches # 0.003 M/sec
( +- 1.20% )
0 cpu-migrations # 0.000 K/sec
( +-100.00% )
167,754 page-faults # 0.003 M/sec
( +- 0.06% )
211,414,701,601 cycles # 4.218 GHz
( +- 4.30% )
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
431,618,818,597 instructions # 2.04 insns per cycle
( +- 6.40% )
80,197,256,524 branches # 1600.165 M/sec
( +- 8.59% )
794,830,352 branch-misses # 0.99% of all branches
( +- 2.05% )
50.177077175 seconds time elapsed
( +- 4.23% )
No improvement (within noise range).
3. x86_64 SPEC06int:
SPEC06int (test set)
[ Y axis: speedup over master ]
8 +-+--+----+----+-----+----+----+----+----+----+----+-----+----+----+--+-+
| |
| tlb-lock-v3 |
7 +-+..................$$$...........................+indirection +-+
| $ $ +resizing |
| $ $ |
6 +-+..................$.$..............................................+-+
| $ $ |
| $ $ |
5 +-+..................$.$..............................................+-+
| $ $ |
| $ $ |
4 +-+..................$.$..............................................+-+
| $ $ |
| +++ $ $ |
3 +-+........$$+.......$.$..............................................+-+
| $$ $ $ |
| $$ $ $ $$$ |
2 +-+........$$........$.$.................................$.$..........+-+
| $$ $ $ $ $ +$$ |
| $$ $$+ $ $ $$$ +$$ $ $ $$$ $$ |
1 +-+***#$***#$+**#$+**#+$**#+$**##$**##$***#$***#$+**#$+**#+$**#+$**##$+-+
| * *#$* *#$ **#$ **# $**# $** #$** #$* *#$* *#$ **#$ **# $**# $** #$ |
| * *#$* *#$ **#$ **# $**# $** #$** #$* *#$* *#$ **#$ **# $**# $** #$ |
0 +-+***#$***#$-**#$-**#$$**#$$**##$**##$***#$***#$-**#$-**#$$**#$$**##$+-+
401.bzi403.gc429445.g456.h462.libq464.h471.omne4483.xalancbgeomean
png: https://imgur.com/a/b1wn3wc
That is, a 1.53x average speedup over master, with a max speedup of 7.13x.
Note that "indirection" (i.e. the first patch in this series) incurs
no overhead, on average.
Signed-off-by: Emilio G. Cota <address@hidden>
---
include/exec/cpu-defs.h | 1 +
accel/tcg/cputlb.c | 36 ++++++++++++++++++++++++++++++++++++
2 files changed, 37 insertions(+)
diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
index 27b9433976..4d1d6b2b8b 100644
--- a/include/exec/cpu-defs.h
+++ b/include/exec/cpu-defs.h
@@ -145,6 +145,7 @@ typedef struct CPUTLBDesc {
size_t size;
size_t mask; /* (.size - 1) << CPU_TLB_ENTRY_BITS for TLB fast path */
size_t used;
+ size_t n_flushes_low_rate;
} CPUTLBDesc;
#define CPU_COMMON_TLB \
diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
index 1ca71ecfc4..afb61e9c2b 100644
--- a/accel/tcg/cputlb.c
+++ b/accel/tcg/cputlb.c
@@ -85,6 +85,7 @@ void tlb_init(CPUState *cpu)
desc->size = MIN_CPU_TLB_SIZE;
desc->mask = (desc->size - 1) << CPU_TLB_ENTRY_BITS;
desc->used = 0;
+ desc->n_flushes_low_rate = 0;
env->tlb_table[i] = g_new(CPUTLBEntry, desc->size);
env->iotlb[i] = g_new0(CPUIOTLBEntry, desc->size);
}
@@ -122,6 +123,39 @@ size_t tlb_flush_count(void)
return count;
}
+/* Call with tlb_lock held */
+static void tlb_mmu_resize_locked(CPUArchState *env, int mmu_idx)
+{
+ CPUTLBDesc *desc = &env->tlb_desc[mmu_idx];
+ size_t rate = desc->used * 100 / desc->size;
+ size_t new_size = desc->size;
+
+ if (rate == 100) {
+ new_size = MIN(desc->size << 2, 1 << TCG_TARGET_TLB_MAX_INDEX_BITS);
+ } else if (rate > 70) {
+ new_size = MIN(desc->size << 1, 1 << TCG_TARGET_TLB_MAX_INDEX_BITS);
+ } else if (rate < 30) {
+ desc->n_flushes_low_rate++;
+ if (desc->n_flushes_low_rate == 100) {
+ new_size = MAX(desc->size >> 1, 1 << MIN_CPU_TLB_BITS);
+ desc->n_flushes_low_rate = 0;
+ }
+ }
+
+ if (new_size == desc->size) {
+ return;
+ }
+
+ g_free(env->tlb_table[mmu_idx]);
+ g_free(env->iotlb[mmu_idx]);
+
+ desc->size = new_size;
+ desc->mask = (desc->size - 1) << CPU_TLB_ENTRY_BITS;
+ desc->n_flushes_low_rate = 0;
+ env->tlb_table[mmu_idx] = g_new(CPUTLBEntry, desc->size);
+ env->iotlb[mmu_idx] = g_new0(CPUIOTLBEntry, desc->size);
+}
+
/* This is OK because CPU architectures generally permit an
* implementation to drop entries from the TLB at any time, so
* flushing more entries than required is only an efficiency issue,
@@ -151,6 +185,7 @@ static void tlb_flush_nocheck(CPUState *cpu)
*/
qemu_spin_lock(&env->tlb_lock);
for (i = 0; i < NB_MMU_MODES; i++) {
+ tlb_mmu_resize_locked(env, i);
memset(env->tlb_table[i], -1,
env->tlb_desc[i].size * sizeof(CPUTLBEntry));
env->tlb_desc[i].used = 0;
@@ -215,6 +250,7 @@ static void tlb_flush_by_mmuidx_async_work(CPUState *cpu,
run_on_cpu_data data)
if (test_bit(mmu_idx, &mmu_idx_bitmask)) {
tlb_debug("%d\n", mmu_idx);
+ tlb_mmu_resize_locked(env, mmu_idx);
memset(env->tlb_table[mmu_idx], -1,
env->tlb_desc[mmu_idx].size * sizeof(CPUTLBEntry));
memset(env->tlb_v_table[mmu_idx], -1, sizeof(env->tlb_v_table[0]));
--
2.17.1
[Qemu-devel] [RFC 1/6] (XXX) cputlb: separate MMU allocation + run-time sizing, Emilio G. Cota, 2018/10/06