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Re: Suggestions for TCG performance improvements

From: Alex Bennée
Subject: Re: Suggestions for TCG performance improvements
Date: Mon, 06 Dec 2021 21:09:38 +0000
User-agent: mu4e 1.7.5; emacs 28.0.90

Vasilev Oleg <vasilev.oleg@huawei.com> writes:

> On 12/3/2021 8:32 PM, Alex Bennée wrote:
>> Vasilev Oleg <vasilev.oleg@huawei.com> writes:
>>> On 12/2/2021 7:02 PM, Alex Bennée wrote:
>>>> Vasilev Oleg <vasilev.oleg@huawei.com> writes:
> ...skipped...
>>>> I did ponder a debug mode which would keep the last N tables dropped by
>>>> tlb_mmu_resize_locked and then measure the differences in the entries
>>>> before submitting the free to an rcu tasks.
>>>>> The mentioned paper[4] also describes other possible improvements.
>>>>> Some of those are already implemented (such as victim TLB and dynamic
>>>>> size for TLB), but others are not (e.g. TLB lookup uninlining and
>>>>> set-associative TLB layer). Do you think those improvements
>>>>> worth trying?
>>>> Anything is worth trying but you would need hard numbers. Also its all
>>>> too easy to target micro benchmarks which might not show much difference
>>>> in real world use. 
>>> The  mentioned paper presents some benchmarking, e. g. linux kernel
>>> compilation and some other stuff. Do you think those shouldn't be
>>> trusted?
>> No they are good. To be honest it's the context switches that get you.
>> Look at "info jit" between a normal distro and a initramfs shell. Places
>> where the kernel is switching between multiple maps means a churn of TLB
>> data.
>> See my other post with a match of "msr ttrb"
> Sorry, couldn't find what you are referring to. Could you, please, share
> a link?

It was an enhancement to the libinsns.so plugin to gauge how often
certain instructions are run:

  Subject: [RFC PATCH  0/2] insn plugin tweaks for measuring frequency
  Date: Fri,  3 Dec 2021 14:44:19 +0000
  Message-Id: <20211203144421.1445232-1-alex.bennee@linaro.org>

I think the msr ttrb[10]_el1 is a key instruction because that triggers
a flush if the ASID changes. On my initramfs setup with a simple login
shell that doesn't happen, on a full distro there is context switching
all the time which causes extra flushes.

Alex Bennée

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