On 13 May '08, at 9:39 PM, Michael Vannorsdel wrote:
If there's some pages of your data you haven't touched in a while the system will swap those out and use the physical pages for something that needs them.
Yes, but it's less efficient than a mapped file, which doesn't have to be swapped out at all.
The OS may have virtual memory, but swapping when the system is under memory pressure is the chief performance problem in OS X; when I worked at Apple, the performance people drilled it into us that the most important optimization is saving memory. (For example, that's why Release builds use -Os by default instead of -O2.)
When making substrings from the data you can save making redundant copies using initWithBytesNoCopy:length:encoding:freeWhenDone: for string objects that only reference the characters from the data. They depend on the NSData object so it needs to be valid for these strings to work.
This is tricky and dangerous. It's very difficult to predict object lifespans in a ref-counted or GC'd environment, and if any of those little strings are still being retained by something when you free the big string, they all turn into land mines that will crash the app the next time they're referenced.
It's possible to make this work, but I would only try it as a last- ditch optimization if the sheer volume of copied strings was choking performance.
(Or -initWithBytesNoCopy: might just copy the bytes anyway. It's only a hint, not a guarantee. I believe it can only use the raw bytes without copying if they're in UTF-16 or ascii or MacRoman encoding.)
—Jens
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