I'm trying to fine tune mmap() to perform fast writes or reads (generally not both) of a potentially very large file. The writes and reads will be mostly sequential on one pass and then likely very sparse on future passes. No region of memory needs to be accessed more than once.
In other words, think of it as a file transfer with some lossiness that gets fixed asynchronously.
It appears, as expected, that the main limitation of mmap()'s performance seems to be the number of minor page faults it generates on large files. Furthermore, I suspect the laziness of the Linux kernel's page-to-disk is causing some performance issues. Namely, any test programs that end up performing huge writes to mmaped memory seem to take a long time after performing all writes to terminate/munmap memory.
I was hoping to offset the cost of these faults by concurrently prefaulting pages while performing the almost-sequential access and paging out pages that I won't need again. But I have three main questions regarding this approach and my understanding of the problem:
MAP_POPULATE flag, but this seems to attempt loading the entire file into memory, which is intolerable in many cases. Also, this seems to cause the mmap() call to block until prefaulting is complete, which is also intolerable. My idea for a manual alternative was to spawn a thread simply to try reading the next N pages in memory to force a prefetch. But it might be that madvise with MADV_SEQUENTIAL already does this, in effect. msync() can be used to flush changes to the disk. However, is it actually useful to do this periodically? My idea is that it might be useful if the program is frequently in an "Idle" state of disk IO and can afford to squeeze in some disk writebacks. Then again, the kernel might very well be handling this itself better than the ever application could. msync() call blocks all disk IO, both to the filesystem cache and to the raw filesystem, then there also isn't as much of an incentive to use it over flushing the entire disk cache at the program's termination.It appears, as expected, that the main limitation of mmap()'s performance seems to be the number of minor page faults it generates on large files.
That's not particularly surprising, I agree. But this is a cost that cannot be avoided, at least for the pages corresponding to regions of the mapped file that you actually access.
Furthermore, I suspect the laziness of the Linux kernel's page-to-disk is causing some performance issues. Namely, any test programs that end up performing huge writes to mmaped memory seem to take a long time after performing all writes to terminate/munmap memory.
That's plausible. Again, this is an unavoidable cost, at least for dirty pages, but you can exercise some influence over when those costs are incurred.
I was hoping to offset the cost of these faults by concurrently prefaulting pages while performing the almost-sequential access and paging out pages that I won't need again. But I have three main questions regarding this approach and my understanding of the problem:
- Is there a straightforward (preferably POSIX [or at least OSX] compatible) way of performing a partial prefault? I am aware of the
MAP_POPULATEflag, but this seems to attempt loading the entire file into memory,
Yes, that's consistent with its documentation.
which is intolerable in many cases. Also, this seems to cause the
mmap()call to block until prefaulting is complete,
That's also as documented.
which is also intolerable. My idea for a manual alternative was to spawn a thread simply to try reading the next N pages in memory to force a prefetch.
Unless there's a delay between when you initially mmap() the file and when you want to start accessing the mapping, it's not clear to me why you would expect that to provide any improvement.
But it might be that madvise with
MADV_SEQUENTIALalready does this, in effect.
If you want POSIX compatibility, then you're looking for posix_madvise(). I would indeed recommend using this function instead of trying to roll your own userspace alternative. In particular, if you use posix_madvise() to assert POSIX_MADV_SEQUENTIAL on some or all of the mapped region, then it is reasonable to hope that the kernel will read ahead to load pages before they are needed. Additionally, if you advise with POSIX_MADV_DONTNEED then you might, at the kernel's discretion, get earlier sync to disk and overall less memory use. There is other advice you can pass by this mechanism, too, if it is useful.
msync()can be used to flush changes to the disk. However, is it actually useful to do this periodically? My idea is that it might be useful if the program is frequently in an "Idle" state of disk IO and can afford to squeeze in some disk writebacks. Then again, the kernel might very well be handling this itself better than the ever application could.
This is something to test. Note that msync() supports asynchronous syncing, however, so you don't need I/O idleness. Thus, when you're sure you're done with a given page you could consider msync()ing it with flag MS_ASYNC to request that the kernel schedule a sync. This might reduce the delay incurred when you unmap the file. You'll have to experiment with combining it with posix_madvise(..., ..., POSIX_MADV_DONTNEED); they might or might not complement each other.
- Is my understanding of disk IO accurate? My assumption is that prefaulting and reading/writing pages can be done concurrently by different threads or processes; if I am wrong about this, then manual prefaulting would not be useful at all.
It should be possible for one thread to prefault pages (by accessing them), while another reads or writes others that have already been faulted in, but it's unclear to me why you expect such a prefaulting thread to be able to run ahead of the one(s) doing the reads and writes. If it has any effect at all (i.e. if the kernel does not prefault on its own) then I would expect prefaulting a page to be more expensive than reading or writing each byte in it once.
Similarly, if an
msync()call blocks all disk IO, both to the filesystem cache and to the raw filesystem, then there also isn't as much of an incentive to use it over flushing the entire disk cache at the program's termination.
There is a minimum number of disk reads and writes that will need to be performed on behalf of your program. For any given mmapped file, they will all be performed on the same I/O device, and therefore they will all be serialized with respect to one another. If you are I/O bound then to a first approximation, the order in which those I/O operations are performed does not matter for overall runtime.
Thus, if the runtime is what you're concerned with, then probably neither posix_madvise() nor msync() will be of much help unless your program spends a significant fraction of its runtime on tasks that are independent of accessing the mmapped file. If you do find yourself not wholly I/O bound then my suggestion would be to see first what posix_madvise() can do for you, and to try asynchronous msync() if you need more. I'm inclined to doubt that userspace prefaulting or synchronous msync() would provide a win, but in optimization, it's always better to test than to (only) predict.