Re: [PATCH -v2 1/2] mm, swap: Use kvzalloc to allocate some swap data structure

[John Hubbard]

On 03/23/2017 07:41 PM, Huang, Ying wrote:
> David Rientjes <rientjes@xxxxxxxxxx> writes:
>
> > On Mon, 20 Mar 2017, Huang, Ying wrote:
> >
> > > From: Huang Ying <ying.huang@xxxxxxxxx>
> > >
> > > Now vzalloc() is used in swap code to allocate various data
> > > structures, such as swap cache, swap slots cache, cluster info, etc.
> > > Because the size may be too large on some system, so that normal
> > > kzalloc() may fail.  But using kzalloc() has some advantages, for
> > > example, less memory fragmentation, less TLB pressure, etc.  So change
> > > the data structure allocation in swap code to use kvzalloc() which
> > > will try kzalloc() firstly, and fallback to vzalloc() if kzalloc()
> > > failed.
> >
> > As questioned in -v1 of this patch, what is the benefit of directly
> > compacting and reclaiming memory for high-order pages by first preferring
> > kmalloc() if this does not require contiguous memory?
>
> The memory allocation here is only for swap on time, not for swap out/in
> time.  The performance of swap on is not considered critical.  But if
> the kmalloc() is used instead of the vmalloc(), the swap out/in
> performance could be improved (marginally).  More importantly, the
> interference for the other activity on the system could be reduced, For
> example, less memory fragmentation, less TLB usage of swap subsystem,
> etc.

Hi Ying,

I'm a little surprised to see vmalloc calls replaced with kmalloc-then-vmalloc calls, because that 
actually makes fragmentation worse (contrary to the above claim). That's because you will consume 
contiguous memory (even though you don't need it to be contiguous), whereas before, you would have 
been able to get by with page-at-a-time for vmalloc.

So, things like THP will find fewer contiguous chunks, as a result of patches such as this.

--
thanks,
john h

> Best Regards,
> Huang, Ying
>
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