sendfile and page usage statistics

David G Lawrence dg at
Wed Oct 10 04:08:10 PDT 2007

> > But I think that the actual patch is wrong. The PG_REFERENCED bit shall
> > be set when hardware access bit is set on pte. You patch would set it in
> > advance.
> The problem is that ethernet card that does TX chsum, DMA and TSO never
> sets PG_A, so sendfile() should set it in advance.
> I'm going to test the patch on machine where all files takes more memory
> than physical memory and there are some popular files.
> I espect to see
> 1) increased active memory.
> 2) decreased number i/o.

   I would say that it is far from clear that this change would result in
better performance. It is generally the case in FreeBSD that we bias in
favor of executable pages (usually the image itself as its pages are
executed, but also the image data pages). Pages involved in regular file
I/O take a back seat. This is because a regular file is accessed typically
in one burst, whereas an executable page is accessed in small chunks over
a much larger period of time (and thus is affected more by a cache miss).
Thus, PG_REFERENCED gets set for pages that are mmaped, but not for
pages involved in read, write, sendfile, etc. Different workloads of
course will have different behavior. 
   One side effect of this will likely be much more system paging activity
as sendfile causes process pages to be pushed out of memory in favor of
cached file data. This is generally a bad thing and would result in reduced
interactive performance.


David G. Lawrence
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