Re: Anticipatory prefaulting in the page fault handler V1

["Martin J. Bligh" <mbligh@aracnet.com>]

> The page fault handler for anonymous pages can generate significant overhead
> apart from its essential function which is to clear and setup a new page
> table entry for a never accessed memory location. This overhead increases
> significantly in an SMP environment.
> 
> In the page table scalability patches, we addressed the issue by changing
> the locking scheme so that multiple fault handlers are able to be processed
> concurrently on multiple cpus. This patch attempts to aggregate multiple
> page faults into a single one. It does that by noting
> anonymous page faults generated in sequence by an application.
> 
> If a fault occurred for page x and is then followed by page x+1 then it may
> be reasonable to expect another page fault at x+2 in the future. If page
> table entries for x+1 and x+2 would be prepared in the fault handling for
> page x+1 then the overhead of taking a fault for x+2 is avoided. However
> page x+2 may never be used and thus we may have increased the rss
> of an application unnecessarily. The swapper will take care of removing
> that page if memory should get tight.
> 
> The following patch makes the anonymous fault handler anticipate future
> faults. For each fault a prediction is made where the fault would occur
> (assuming linear acccess by the application). If the prediction turns out to
> be right (next fault is where expected) then a number of pages is
> preallocated in order to avoid a series of future faults. The order of the
> preallocation increases by the power of two for each success in sequence.
> 
> The first successful prediction leads to an additional page being allocated.
> Second successful prediction leads to 2 additional pages being allocated.
> Third to 4 pages and so on. The max order is 3 by default. In a large
> continous allocation the number of faults is reduced by a factor of 8.
> 
> The patch may be combined with the page fault scalability patch (another
> edition of the patch is needed which will be forthcoming after the
> page fault scalability patch has been included). The combined patches
> will triple the possible page fault rate from ~1 mio faults sec to 3 mio
> faults sec.
> 
> Standard Kernel on a 512 Cpu machine allocating 32GB with an increasing
> number of threads (and thus increasing parallellism of page faults):

Mmmm ... we tried doing this before for filebacked pages by sniffing the
pagecache, but it crippled forky workloads (like kernel compile) with the 
extra cost in zap_pte_range, etc. 

Perhaps the locality is better for the anon stuff, but the cost is also
higher. Exactly what benchmark were you running on this? If you just run
a microbenchmark that allocates memory, then it will definitely be faster.
On other things, I suspect not ...

M.