> On Jul 14, 2017, at 2:35 PM, John McCall <rjmcc...@apple.com> wrote: > >> On Jul 14, 2017, at 1:12 PM, Charles Srstka via swift-evolution >> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >> MOTIVATION: >> >> Meet Bob. Bob is a developer with mostly C++ and Java experience, but who >> has been learning Swift. Bob needs to write an app to parse some proprietary >> binary data format that his company requires. Bob’s written this app, and >> it’s worked pretty well on Linux: >> >> import Foundation >> >> do { >> let url = ... >> >> let handle = try FileHandle(forReadingFrom: url) >> let bufsize = 1024 * 1024 // read 1 MiB at a time >> >> while true { >> let data = handle.readData(ofLength: bufsize) >> >> if data.isEmpty { >> break >> } >> >> data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) in >> // do something with bytes >> } >> } >> } catch { >> print("Error occurred: \(error.localizedDescription)") >> } >> >> Later, Bob needs to port this same app to macOS. All seems to work well, >> until Bob tries opening a large file of many gigabytes in size. Suddenly, >> the simple act of running the app causes Bob’s Mac to completely lock up, >> beachball, and finally pop up with the dreaded “This computer is out of >> system memory” message. If Bob’s particularly unlucky, things will locked up >> tight enough that he can’t even recover from there, and may have to >> hard-reboot the machine. >> >> What happened? >> >> Experienced Objective-C developers will spot the problem right away; the >> Foundation APIs that Bob used generated autoreleased objects, which would >> never be released until Bob’s loop finished. However, Bob’s never programmed >> in Objective-C, and to him, this behavior is completely undecipherable. >> >> After a copious amount of time spent Googling for answers and asking for >> help on various mailing lists and message boards, Bob finally gets the >> recommendation from someone to try wrapping the file handle read in an >> autorelease pool. So he does: >> >> import Foundation >> >> do { >> let url = ... >> >> let handle = try FileHandle(forReadingFrom: url) >> let bufsize = 1024 * 1024 // read 1 MiB at a time >> >> while true { >> let data = autoreleasepool { handle.readData(ofLength: bufsize) } >> >> if data.isEmpty { >> break >> } >> >> data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) in >> // do something with bytes >> } >> } >> } catch { >> print("Error occurred: \(error.localizedDescription)") >> } >> >> Unfortunately, Bob’s program still eats RAM like Homer Simpson in an >> all-you-can-eat buffet. Turns out the data.withUnsafeBytes call *also* >> causes the data to be autoreleased. > > This seems like a bug that should be fixed. I don't know why the other one > would cause an unreclaimable autorelease.
Sticking a break at the end of my original code snippet so the loop runs only once and then running it through Instruments, it seems the NSConcreteData instance gets autoreleased… 32,769 times. o_O First autorelease occurs inside -[NSConcreteFileHandle readDataOfLength:], the next 32,768 occur in Data.Iterator.next(), which is called by specialized RangeReplaceableCollection.init<A>. I can send you the trace off-list if you’d like. Charles
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