linux-kernel - Re: [RFC 1/3] mm: add thp_utilization metrics to debugfs

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Message-ID: <4847C6FE-429D-4619-9200-6CE7C84B5386@fb.com>
Date:   Mon, 29 Aug 2022 20:19:26 +0000
From:   "Alex Zhu (Kernel)" <alexlzhu@...com>
To:     Zi Yan <ziy@...dia.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Matthew Wilcox <willy@...radead.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "riel@...riel.com" <riel@...riel.com>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/3] mm: add thp_utilization metrics to debugfs

> On Aug 26, 2022, at 5:11 PM, Zi Yan <ziy@...dia.com> wrote:
> 
> On 25 Aug 2022, at 17:30, alexlzhu@...com wrote:
> 
> How large is the memory? Just wonder the scanning speed.
> Also, it might be better to explicitly add the time unit, second,
> in the output.

The size of memory was 65GB on the test machine I obtained these 
numbers on. I’ll take note of adding the time unit. Thanks!

> Is it possible to use cache-bypassing read to avoid cache
> pollution? You are scanning for 256*2M at a time. Wouldn’t that
> wipe out all the useful data in the cache?

I have only found non-temporal writes in arch/x86/, not non-temporal reads (with MOVNTDQA). I suppose we should figure out why nobody ever bothered using non-temporal reads on x86 before trying to make this code use them.

A quick search of the internet suggests they non-temporal reads are not being used on x86 because people could not show a performance improvement by using them, but maybe somebody here has more insight?