Windows相当于Linux的readahead系统调用？

Question

Is there a Windows equivalent to Linux's readahead syscall?

EDIT:

I would like a full function signature if possible, showing the equivalent offset/count parameters (or lower/upper).

Eg: The Linux function signature is:

ssize_t readahead(int fd, off64_t *offset, size_t count);

and an example of it's use is

readahead(file, 100, 500);

Where "file" is a file descriptor previously set by a function like mmap. This call is reading 500 bytes at index 100.

EDIT 2: Please read this if you are unsure what readahead does: http://linux.die.net/man/2/readahead

Answer 1

Yes. It is FileSystemControl FSCTL_FILE_PREFETCH .

It is used in Windows Vista and above for prefetching both when an application starts and at boot time.

It is also used by the SuperFetch technology that uses heuristics to load applications at approximately the times of day you generally use them.

FSCTL_FILE_PREFETCH itself is not documented on MSDN, but it is easy to figure out the parameter format by examining the DeviceIoControl calls made on app startup: Just start an application in the debugger that already has a .pf file in the c:\\Windows\\Prefetch directory and break on DeviceIoControl (or if you're using a kernel debugger, break when the NTFS driver receives its first FSCTL_FILE_PREFETCH). Examine the buffer passed in and compare it with the .pf file and the range actually used later. I did this once out of curiosity but didn't record the details.

In case you are unfamiliar with DeviceIoControl and IRP_MJ_FILESYSTEM_CONTROL , here are some links to look at:

• FileSystemControl at the IRP level IRP_MJ_FILESYSTEM_CONTROL
• DeviceIoControl , which is used to invoke FileSystemControl IRPs
• Structure of IO Control Codes

Answer 2

As of Windows 8, there exists a more or less direct equivalent to madvise(MADV_WILLNEED) , which is effectively the same thing (Windows has an unified VM/cache system).

Assuming that you have memory-mapped the file, you can thus use PrefetchVirtualMemory to prefetch it.

This is still slightly more complicated than you'd wish, but not nearly as harsh as DeviceIoControl . Also note that you can easily prefetch several independent, discontinuous ranges.

Answer 3

I am not sure if I understand correctly, in what you said ' Where "file" is a file descriptor previously set by a function like mmap. This call is reading 500 bytes at index 100. ' That sounds suspiciously like seeking to the offset and read 500 bytes... but you want it to be pre-fetched ahead...

In C Code, it would look something like this:

fseek(fp, 100, SEEK_CUR);
fread(&data, 500, 1, fp);

But prefetching it, I guess, you would want to hook up some kind of events using waithandles, and when the event gets raised, the data gets stored somewhere in a buffer...

To be honest, I have not come across a such thing that does pre-fetching data...but Ray's answer surprised me, but then again it is only for Vista upwards, if you want to maintain compatibility...that's something to keep in mind... but the links below may be of help...

Ok, there was a blog discussing this, a library written in Delphi, the source code is here , browsing the code here , ok, it may not be what you want but it may help you point in the direction... Sorry if its not what you are looking for...

Hope this helps, Best regards, Tom.