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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Tue, 10 Oct 2023 23:39:44 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Raghavendra K T <raghavendra.kt@amd.com>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        Bharata B Rao <bharata@amd.com>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 6/6] sched/numa: Complete scanning of inactive VMAs when
 there is no alternative
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* Mel Gorman <mgorman@techsingularity.net> wrote:

> > 64 (BITS_PER_LONG) feels a bit small, especially on larger machines running 
> > threaded workloads, and the kmalloc of numab_state likely allocates a full 
> > cacheline anyway, so we could double the hash size from 8 bytes (2x1 longs) 
                                                            ^--- 16 bytes
> > to 32 bytes (2x2 longs) with very little real cost, and still have a long 
> > field left to spare?
> > 
> 
> You're right, we could and it's relatively cheap. I would worry that as 
> the storage overhead is per-VMA then workloads for large machines may 
> also have lots of VMAs that are not necessarily using threads.

So I think there would be *zero* extra per-vma storage overhead, because 
vma->numab_state is a pointer, with the structure kmalloc() allocated, 
which should round the allocation to cacheline granularity anyway: 64 bytes 
on NUMA systems that matter.

So with the current size of 'struct vma_numab_state' of 36 bytes, we can 
extend it by 16 bytes with zero additional storage cost.

And since there's no cost, and less hash collisions are always good, the 
change wouldn't need any other justification. :-)

[ Plus the resulting abstraction for the definition of a larger bitmask 
  would probably make future extensions easier. ]

But ... it was just a suggestion.

Thanks,

	Ingo