Reading NSTask output persistent memory usage - objective-c

Question

I have a simple NSTask which runs a shell command, and a NSPipe file handle for reading which is used to read the output of the command and writes out to a file. When this command terminates and the output is written to file, Activity Monitor and Xcode memory monitor shows 700mb of usage which never goes down. I have wrapped the NSData output in an autoreleasepool and the whole task is in it's own function so there is no reason I can see for the high usage when the task is finished. I have profiled this with instruments and there are apparently no leaks and only a few KB of persistent memory after the command runs.
- (void)extractLogs forDate:(NSDate *)date
{
__block NSError* err;
NSDate *collectionEndDate = [[NSCalendar currentCalendar]dateByAddingUnit:NSCalendarUnitDay
value:1
toDate:_collectionStartDate
options:0];
[NSString stringWithFormat:#"%#",collectionEndDate];
NSPipe *pipe = [NSPipe new];
NSTask *logRetrievalTask = [[NSTask alloc]init];
[logRetrievalTask setLaunchPath:#"/bin/bash"];
NSString *cmd = [[NSString alloc]initWithFormat:#"/usr/bin/log show --signpost --no-pager --source --style syslog --start %# --end %# --predicate \"%#\" | %#",
[_collectionStartDate logFormattedDateString],
[collectionEndDate logFormattedDateString],
[NSString stringWithFormat:#"subsystem == '%#'", "com.xxx.xxxxx"],
LOG_TRANSFORM_COMMAND];
[logRetrievalTask setArguments:#[#"-c",
cmd]];
[logRetrievalTask setStandardOutput:pipe];
NSURL *logFileUrl = [self logFilePathForSubsystem:system usingDateString:[_collectionStartDate sduLogFormattedDateString]];
__block FileStreamCompressor *compressor = [[FileStreamCompressor alloc]
initWithFilePath:[logFileUrl path]
maxFileSize:MB_IN_BYTES([self maxFileSize])
currentTotalBytes: _totalLogBytes];
if (compressor == nil)
{
[[self logger]indicateFailureWith:[NSString stringWithFormat:#"Error: Could not create log file for: %#", system]];
return;
}
[logRetrievalTask launch];
NSData *logData = [[pipe fileHandleForReading]readDataToEndOfFile];
[logRetrievalTask terminate];
if ([logData length] == LOG_COMMAND_OUTPUT_HEADER_SIZE)
{
[compressor finishWritingStreamToFile];
[self removeLogFileForSystem:system
usingDateString:[_collectionStartDate logFormattedDateString]
logFileErrorType:#"Empty log file"];
} else {
_totalLogBytes = [compressor compressData:logData error:&err];
if (err != nil) {
[compressor finishWritingStreamToFile];
[[self logger]indicateWarningWith:[err localizedDescription]];
[self removeLogFileForSystem:system
usingDateString:[_collectionStartDate logFormattedDateString]
logFileErrorType:#"Corrupt logfile"];
} else {
[[self logger]indicateSuccessWith:[NSString stringWithFormat:#"Finished processing log: %#-%#.log.gz",
system,
[_collectionStartDate logFormattedDateString]]];
}
}
[[pipe fileHandleForReading]closeFile];
[[pipe fileHandleForWriting]closeFile];
}