On Sun, Jan 30, 2011 at 6:32 AM, Steven Watanabe <watanabesj@gmail.com> wrote:
AMDG
On 1/29/2011 1:18 PM, Dean Michael Berris wrote:
On Sun, Jan 30, 2011 at 4:39 AM, Steven Watanabe<watanabesj@gmail.com> wrote:
On 1/29/2011 1:25 AM, Dean Michael Berris wrote:
-- which means the VMM no matter what you do will actually have to find the memory for you and give you that chunk of memory. This means potentially swapping pages in/out. You limit this likelihood by asking for page-sized and page-aligned chunks and using those properly;
Why?
Why will page-sized/page-aligned chunks avoid the likelihood of (unnecessarily) swapping pages in and out? Because if you didn't take care that your data would spill over to page boundaries the amount of pages you're touching is one more than you need to touch.
One more comment, and then I'm done with this thread. I agree that page-alignment will help reduce swapping. What I remain unconvinced about are the benefits of allocating only single pages.
Right. When I say "allocate", I use it as in "ask a C++ allocator to give me a chunk" -- as in call the alloca member of the allocator instance. The thought really is prudence: ask only for what you need and make the most of what you have. In the string context: For concatenation nodes, laying it out in a page-aligned/aware manner and packing as much of them in a single page as possible is generally a good thing -- better than if you just willy-nilly allocate from anywhere or without regard for page boundaries. For block nodes, this means having a "usable block" that can be shared by multiple strings and referred to by concatenation nodes. In general, since you cannot control the growth factor for heap spaces generally from within a C++ application, using predictably sized units (like page sizes worth of memory) for *segmented* data structures is a good thing. Again if you knew beforehand that you need contiguous memory *and* don't really care about page alignment then you don't need this scheme. When growing segmented data structures having a predictable (preferably constant) growth characteristic or segment size is usually a good approach. This requires cooperation from an allocator that knows how to reserve/grow/shrink appropriately sized memory that is again page-aligned for maximum efficiency. HTH FWIW, I'm done with this thread too now. -- Dean Michael Berris about.me/deanberris