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Re: [RFC PATCH 0/5] Experimenting with tb-lookup tweaks
From: |
Alex Bennée |
Subject: |
Re: [RFC PATCH 0/5] Experimenting with tb-lookup tweaks |
Date: |
Thu, 25 Feb 2021 10:15:42 +0000 |
User-agent: |
mu4e 1.5.8; emacs 28.0.50 |
Richard Henderson <richard.henderson@linaro.org> writes:
> On 2/24/21 8:58 AM, Alex Bennée wrote:
>> Hi Richard,
>>
>> Well I spun up some of the ideas we talked about to see if there was
>> anything to be squeezed out of the function. In the end the results
>> seem to be a washout with my pigz benchmark:
>>
>> qemu-system-aarch64 -cpu cortex-a57 \
>> -machine type=virt,virtualization=on,gic-version=3 \
>> -serial mon:stdio \
>> -netdev user,id=unet,hostfwd=tcp::2222-:22 \
>> -device virtio-net-pci,netdev=unet,id=virt-net,disable-legacy=on \
>> -device virtio-scsi-pci,id=virt-scsi,disable-legacy=on \
>> -blockdev
>> driver=raw,node-name=hd,discard=unmap,file.driver=host_device,file.filename=/dev/zen-disk/debian-buster-arm64
>> \
>> -device scsi-hd,drive=hd,id=virt-scsi-hd \
>> -smp 4 -m 4096 \
>> -kernel ~/lsrc/linux.git/builds/arm64/arch/arm64/boot/Image \
>> -append "root=/dev/sda2 systemd.unit=benchmark-pigz.service" \
>> -display none -snapshot
>>
>> | Command | Mean [s] | Min [s] | Max [s] | Relative |
>> |---------+----------------+---------+---------+----------|
>> | Before | 46.597 ± 2.482 | 45.208 | 53.618 | 1.00 |
>> | After | 46.867 ± 2.242 | 45.871 | 53.180 | 1.00 |
>
> Well that's disappointing.
>
>> Maybe the code cleanup itself makes it worthwhile. WDYT?
>
> I think there's little doubt that the first 3 patches are a good code cleanup.
>
> Patch 4 I think is still beneficial, simply so that we can add that "Above
> fields" comment.
>
> Patch 5 would only be worthwhile if we could measure any positive difference,
> which it seems we cannot.
>
> I have a follow-up patch to remove the parallel_cpus global variable which I
> will post in a moment. While it removes a handful of insns from this
> fast-path, I doubt it helps. But getting rid of a global is probably always
> positive, no?
>
> I was glancing through the lookup function for alpha, instead of aarch64 and
> saw:
>
> 21e: 33 43 18 xor 0x18(%rbx),%eax
> 221: 4c 31 e1 xor %r12,%rcx
> 224: 44 31 ea xor %r13d,%edx
> 227: 09 c2 or %eax,%edx
> 229: 48 0b 4b 08 or 0x8(%rbx),%rcx
>
> and thought -- hang on, how come we're just ORing nor XORing here? Of course
> it's the cs_base field, which alpha has set to zero. The compiler has
> simplified bits |= 0 ^ tb->cs_base.
>
> Which got me thinking: what if we had a per-cpu
>
> typedef struct {
> target_ulong pc;
> ...
> } TranslationBlockID;
>
> static inline bool arch_tbid_cmp(TranslationBlockID x,
> TranslationBlockID y)
> {
> return x.pc == y.pc && ...;
> }
>
> We could potentially reduce this to memcmp(&x, &y).
>
> First, this would allow cs_base to be eliminated where it is not used.
> Second,
> this would allow cs_base to be renamed for the non-x86 targets for which it is
> being abused. Third, it would allow tb->flags to be either (a) elided or (b)
> extended by the target as needed.
>
> This final is directed at ARM, of course, where we've overflowed the uint32_t
> that is tb->flags. We could now extend that to 64-bits.
>
> Obviously, some tweaks to tb_hash_func would be required as well, but that's
> manageable.
>
> What do you think about this last?
Sounds like a good idea for clean-up, especially to get rid of
cs_base/extend tbflags when needed. One concern would be where do we go
when we get to heterogeneous emulation? Will they share the same
translation area like the current cpu->cluster_index stuff or will that
only be for similar but not quite the same architectures? Maybe I'm
thinking too far ahead...
>
>
> r~
--
Alex Bennée
- [RFC PATCH 0/5] Experimenting with tb-lookup tweaks, Alex Bennée, 2021/02/24
- [RFC PATCH 4/5] include/exec: lightly re-arrange TranslationBlock, Alex Bennée, 2021/02/24
- [RFC PATCH 5/5] include/exec/tb-lookup: try and reduce branch prediction issues, Alex Bennée, 2021/02/24
- [RFC PATCH 1/5] accel/tcg: rename tb_lookup__cpu_state and hoist state extraction, Alex Bennée, 2021/02/24
- [RFC PATCH 2/5] accel/tcg: move CF_CLUSTER calculation to curr_cflags, Alex Bennée, 2021/02/24
- [RFC PATCH 3/5] accel/tcg: drop the use of CF_HASH_MASK and rename params, Alex Bennée, 2021/02/24
- Re: [RFC PATCH 0/5] Experimenting with tb-lookup tweaks, Richard Henderson, 2021/02/24
- Re: [RFC PATCH 0/5] Experimenting with tb-lookup tweaks,
Alex Bennée <=
- Re: [RFC PATCH 0/5] Experimenting with tb-lookup tweaks, no-reply, 2021/02/25