Re: [patch] cache pipe buf page address for non-highmem arch

[Christoph Hellwig <hch@infradead.org>]

On Thu, Mar 22, 2007 at 05:51:11PM -0700, Ken Chen wrote:
> It is really sad that we always call kmap and friends for every pipe
> buffer page on 64-bit arch that doesn't use HIGHMEM, or on
> configuration that doesn't turn on HIGHMEM.
> 
> The effect of calling kmap* is visible in the execution profile when
> pipe code is being stressed.  It is especially true on amd's x86-64
> platform where kmap() has to traverse through numa node index
> calculation in order to convert struct page * to kernel virtual
> address.  It is fairly pointless to perform that calculation repeatly
> on system with no highmem (i.e., 64-bit arch like x86-64).  This patch
> caches kernel pipe buffer page's kernel vaddr to speed up pipe buffer
> mapping functions.
> 
> There is another suboptimal block in pipe_read() where wake_up is
> called twice.  I think it was an oversight since in pipe_write(), it
> looks like it is doing the right thing.

I think you're fixing the symptom here and not the cause.  If calculating
the virtual address of a page is so expensive on your setup it should
define WANT_PAGE_VIRTUAL and we should always cache the virtual address
in struct page.  There's a lot more code, epecially in filesystems that's
rather upset about a slow page_address.