Re: [RFC PATCH 0/5] hugetlb: Change huge pmd sharing

[David Hildenbrand]

>>
>> Let's assume a 4 TiB device and 2 MiB hugepage size. That's 2097152 huge
>> pages. Each such PMD entry consumes 8 bytes. That's 16 MiB.
>>
>> Sure, with thousands of processes sharing that memory, the size of page
>> tables required would increase with each and every process. But TBH,
>> that's in no way different to other file systems where we're even
>> dealing with PTE tables.
> 
> The numbers for a real use case I am frequently quoted are something like:
> 1TB shared mapping, 10,000 processes sharing the mapping
> 4K PMD Page per 1GB of shared mapping
> 4M saving for each shared process
> 9,999 * 4M ~= 39GB savings

3.7 % of all memory. Noticeable if the feature is removed? yes. Do we
care about supporting such corner cases that result in a maintenance
burden? My take is a clear no.

> 
> However, if you look at commit 39dde65c9940c which introduced huge pmd sharing
> it states that performance rather than memory savings was the primary
> objective.
> 
> "For hugetlb, the saving on page table memory is not the primary
>  objective (as hugetlb itself already cuts down page table overhead
>  significantly), instead, the purpose of using shared page table on hugetlb is
>  to allow faster TLB refill and smaller cache pollution upon TLB miss.
>     
>  With PT sharing, pte entries are shared among hundreds of processes, the
>  cache consumption used by all the page table is smaller and in return,
>  application gets much higher cache hit ratio.  One other effect is that
>  cache hit ratio with hardware page walker hitting on pte in cache will be
>  higher and this helps to reduce tlb miss latency.  These two effects
>  contribute to higher application performance."
> 
> That 'makes sense', but I have never tried to measure any such performance
> benefit.  It is easier to calculate the memory savings.

It does makes sense; but then, again, what's specific here about hugetlb?

Most probably it was just easy to add to hugetlb in contrast to other
types of shared memory.

> 
>>
>> Which results in me wondering if
>>
>> a) We should simply use gigantic pages for such extreme use case. Allows
>>    for freeing up more memory via vmemmap either way.
> 
> The only problem with this is that many processors in use today have
> limited TLB entries for gigantic pages.
> 
>> b) We should instead look into reclaiming reconstruct-able page table.
>>    It's hard to imagine that each and every process accesses each and
>>    every part of the gigantic file all of the time.
>> c) We should instead establish a more generic page table sharing
>>    mechanism.
> 
> Yes.  I think that is the direction taken by mshare() proposal.  If we have
> a more generic approach we can certainly start deprecating hugetlb pmd
> sharing.

My strong opinion is to remove it ASAP and get something proper into place.

-- 
Thanks,

David / dhildenb