Re: [Qemu-devel] [RFC 6/6] cputlb: dynamically resize TLBs based on use rate
On Sun, Oct 07, 2018 at 21:48:34 -0400, Emilio G. Cota wrote: > - 70/40% use rate for growing/shrinking the TLB does not > seem a great choice, if one wants to avoid a pathological > case that can induce constant resizing. Imagine we got > exactly 70% use rate, and all TLB misses were compulsory > (i.e. a direct-mapped TLB would have not prevented a ^^^ > single miss). We'd then double the TLB size: I meant fully associative. E.
Re: [Qemu-devel] [RFC 6/6] cputlb: dynamically resize TLBs based on use rate
On 10/6/18 2:45 PM, Emilio G. Cota wrote:
> @@ -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 = >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) {
s/desc->size/old_size/g
Otherwise it looks plausible as a first cut.
r~
Re: [Qemu-devel] [RFC 6/6] cputlb: dynamically resize TLBs based on use rate
On Sun, Oct 07, 2018 at 19:37:50 +0200, Philippe Mathieu-Daudé wrote:
> On 10/6/18 11:45 PM, Emilio G. Cota wrote:
> > 2. System boot + shutdown, ubuntu 18.04 x86_64:
>
> You can also run the VM tests to build QEMU:
>
> $ make vm-test
Thanks, will give that a look.
> > +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) {
>
> I wonder if those thresholds might be per TCG_TARGET.
Do you mean to tune the growth rate based on each TCG target?
(max and min are already determined by the TCG target).
The optimal growth rate is mostly dependent on the guest
workload, so I wouldn't expect the TCG target to matter
much.
That said, we could spend quite some time tweaking the
TLB sizing algorithm. But with this RFC I wanted to see
(a) whether this approach is a good idea at all, and (b) show
what 'easy' speedups might look like (because converting
all TCG targets is a pain, so it better be justified).
> Btw the paper used 40% here, did you tried it too?
Yes, I tried several alternatives including what the
paper describes, i.e. (skipping the min/max checks
for simplicity):
if (rate > 70) {
new_size = 2 * old_size;
} else if (rate < 40) {
new_size = old_size / 2;
}
But that didn't give great speedups (see "resizing-paper"
set):
https://imgur.com/a/w3AqHP7
A few points stand out to me:
- We get very different speedups even if we implement
the algorithm they describe (not sure that's exactly
what they implemented though). But there are many
variables that could explain that, e.g. different guest
images (and therefore different TLB flush rates) and
different QEMU baselines (ours is faster than the paper's,
so getting speedups is harder).
- 70/40% use rate for growing/shrinking the TLB does not
seem a great choice, if one wants to avoid a pathological
case that can induce constant resizing. Imagine we got
exactly 70% use rate, and all TLB misses were compulsory
(i.e. a direct-mapped TLB would have not prevented a
single miss). We'd then double the TLB size:
size_new = 2*size_old
But then the use rate will halve:
use_new = 0.7/2 = 0.35
So we'd then end up in a grow-shrink loop!
Picking a "shrink threshold" below 0.70/2=0.35 avoids this.
- Aggressively increasing the TLB size when usage is high
makes sense. However, reducing the size at the same
rate does not make much sense. Imagine the following
scenario with two processes being scheduled: one process
uses a lot of memory, and the other one uses little, but
both are CPU-intensive and therefore being assigned similar
time slices by the scheduler. Ideally you'd resize the TLB
to meet each process' memory demands. However, at flush
time we don't even know what process is running or about
to run, so we have to size the TLB exclusively based on
recent use rates. In this scenario you're probably close
to optimal if you size the TLB to meet the demands of the
most memory-hungry process. You'll lose some extra time
flushing the (now larger) TLB, but your net gain is likely
to be positive given the TLB fills you won't have to do
when the memory-heavy process is scheduled in.
So to me it's quite likely that in the paper they
could have gotten even better results by reducing the
shrink rate, like we did.
Thanks,
Emilio
Re: [Qemu-devel] [RFC 6/6] cputlb: dynamically resize TLBs based on use rate
Hi Emilio, On 10/6/18 11:45 PM, Emilio G. Cota wrote: > 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: You can also run the VM tests to build QEMU: $ make vm-test vm-test: Test QEMU in preconfigured virtual machines vm-build-ubuntu.i386- Build QEMU in ubuntu i386 VM vm-build-freebsd- Build QEMU in FreeBSD VM vm-build-netbsd - Build QEMU in NetBSD VM vm-build-openbsd- Build QEMU in OpenBSD VM vm-build-centos - Build QEMU in CentOS VM, with Docker > > - 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% ) > stalled-cycles-frontend > 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 +-+ > |$ $
[Qemu-devel] [RFC 6/6] cputlb: dynamically resize TLBs based on use rate
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% ) stalled-cycles-frontend 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 | |$ $| 6 +-+..$.$..+-+ |$ $| |$ $| 5 +-+..$.$..+-+ |$ $| |$ $| 4 +-+..$.$..+-+ |$ $
