Re: How to find out which pages were copied-on-write?

[Michael Clark]

On 07/13/04 21:04, Lutz Vieweg wrote:
> > You don't use mmap for speed but rather for convenience.
>
>
> But isn't an advantage with mmap() that there's no need for the kernel
> to copy what is to be written to a dedicated buffer? The kernel
> could initiate DMA writes directly from the working memory...

Yes, but page faults are expensive too. Each time a page is written
out it needs to be marked read only again and will cause a page fault
for the next write access from userspace. For certain workloads this
can easily add up to more than copy_(to|from)_user in read/write.

read/write also gives you more explicit control on IO batching and
scheduling (when to read or write). Less need for the kernel to employ
tricks to effectively coaslesce IOs on dirtied pages or sense
streaming access patterns.

~mc
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