Messages in this thread | | | Date | Fri, 10 Oct 2003 21:03:38 +0200 | From | Andrea Arcangeli <> | Subject | Re: statfs() / statvfs() syscall ballsup... |
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On Fri, Oct 10, 2003 at 11:36:29AM -0700, Linus Torvalds wrote: > > On Fri, 10 Oct 2003, Andrea Arcangeli wrote: > > > > O_DIRECT only walk the pagetables, no pte mangling, no tlb flushes, the > > TLB is preserved fully. > > Yes. However, it's even _nicer_ if you don't need to walk the page tables > at all. > > Quite a lot of operations could be done directly on the page cache. I'm > not a huge fan of mmap() myself - the biggest advantage of mmap is when > you don't know your access patterns, and you have reasonably good > locality. In many other cases mmap is just a total loss, because the page > table walking is often more expensive than even a memcpy(). > > That's _especially_ true if you have to move mappings around, and you have > to invalidate TLB's.
agreed. that's what remap_file_pages does infact.
> memcpy() often gets a bad name. Yeah, memory is slow, but especially if > you copy something you just worked on, you're actually often better off > letting the CPU cache do its job, rather than walking page tables and > trying to be clever. > > Just as an example: copying often means that you don't need nearly as much > locking and synchronization - which in turn avoids one whole big mess > (yes, the memcpy() will look very hot in profiles, but then doing extra > work to avoid the memcpy() will cause spread-out overhead that is a lot > worse and harder to think about). > > This is why a simple read()/write() loop often _beats_ mmap approaches. > And often it's actually better to not even have big buffers (ie the old > "avoid system calls by aggregation" approach) because that just blows your > cache away. > > Right now, the fastest way to copy a file is apparently by doing lots of > ~8kB read/write pairs (that data may be slightly stale, but it was true at > some point). Never mind the system call overhead - just having the extra > buffer stay in the L1 cache and avoiding page faults from mmap is a bigger > win. > > And I don't think mmap _can_ beat that. It's fundamental.
That's my whole point, agreed. Though using mmap would be sure cleaner and simpler.
> In contrast, direct page cache accesses really can do so. Exactly because > they don't touch any page tables at all, and because they can take > advantage of internal kernel data structure layout and move pages around > without any cost..
Which basically means removing O_DIRECT from the open syscalls and still use read/write if I understand correctly.
With todays commodity dirtcheap hardware, it has been proven that walking the pte (NOTE: only walking, no mangling and no tlb flushing) is much faster than doing the memcpy. More cpu is left free for the other tasks and the cost of the I/O is the same. The different isn't measurable in I/O bound tasks, but a database is both IO bound and cpu bound at the same time, so for a db it's measurable. At least this is the case for Oracle. I believe Joel has access to these numbers too, and that's why he's interested in O_DIRECT in the first place.
With faster membus things may change of course (to the point where there's no difference between the two models), but still I don't see how can walking tree pointers to be more expensive than copying 512bytes of data (assuming the smaller blocksize). And you're ignoring the CPU *has* to walk those three pointers _anyways_ implicitly to allow the memcpy to run. So as far as I can tell the memcpy is pure overhead that can be avoided with O_DIRECT.
this is also why I rejected all approcches that wanted to allow readahead via O_DIRECT by preloading data in pagecache, my argument is: if you can't avoid the memcpy you must not use O_DIRECT. The only signle object of O_DIRECT is to avoid the memcpy, the cache pollution avoidance is a very minor issue, the main point is to avoid the memcpy.
I also posted a number of benchmarks at some point, where I've shown a dramatical reduction of the cpu usage, up to 10% reduction, on a normal cheap hardware w/o reduction of I/O bandwidth. This means 10% more cpu to use for doing something useful in the cpu bound part of the database.
The main downside of O_DIRECT I believe conceptual, starting from the ugliness inside the kernel, like the cache coherency handling and i_alloc_sem need to avoid reads to run in parallel of block allocations, etc... but the practical effect I doubt can be easily beaten in the numbers. That said maybe we can provide a nicer API that does the same thing internally I don't know, but certainly that can't be remap_file_pages because that does a very different thing.
Andrea - If you prefer relying on open source software, check these links: rsync.kernel.org::pub/scm/linux/kernel/bkcvs/linux-2.[45]/ http://www.cobite.com/cvsps/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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