Messages in this thread | | | Date | Fri, 23 Apr 2010 19:05:54 +0200 | From | Jens Axboe <> | Subject | Re: Trying to measure performance with splice/vmsplice .... |
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On Fri, Apr 23 2010, Steven J. Magnani wrote: > On Fri, 2010-04-23 at 09:07 -0700, Rick Sherm wrote: > > Hello Jens - any assistance/pointers on 1) and 2) below > > will be great.I'm willing to test out any sample patch. > > Recent mail from him has come from jens.axboe@oracle.com, I cc'd it.
Goes to the same inbox in the end, so no difference :-)
> > > On Fri, 2010-04-16 at 10:02 -0700, Rick Sherm wrote: > > > > Q3) When using splice, even though the destination > > > file is opened in O_DIRECT mode, the data gets cached. I > > > verified it using vmstat. > > > > > > > > r b swpd free buff cache > > > > 1 0 0 9358820 116576 2100904 > > > > > > > > ./splice_to_splice > > > > > > > > r b swpd free buff cache > > > > 2 0 0 7228908 116576 4198164 > > > > > > > > I see the same caching issue even if I vmsplice > > > buffers(simple malloc'd iov) to a pipe and then splice the > > > pipe to a file. The speed is still an issue with vmsplice > > > too. > > > > > > > > > > One thing is that O_DIRECT is a hint; not all filesystems > > > bypass the cache. I'm pretty sure ext2 does, and I know fat doesn't. > > > > > > Another variable is whether (and how) your filesystem > > > implements the splice_write file operation. The generic one (pipe_to_file) > > > in fs/splice.c copies data to pagecache. The default one goes > > > out to vfs_write() and might stand more of a chance of honoring > > > O_DIRECT. > > > > > > > True.I guess I should have looked harder. It's xfs and xfs's->file_ops points to 'generic_file_splice_read[write]'.Last time I had to 'fdatasync' and then fadvise to mimic 'O_DIRECT'. > > > > > > Q4) Also, using splice, you can only transfer 64K > > > worth of data(PIPE_BUFFERS*PAGE_SIZE) at a time,correct?.But > > > using stock read/write, I can go upto 1MB buffer. After that > > > I don't see any gain. But still the reduction in system/cpu > > > time is significant. > > > > > > I'm not a splicing expert but I did spend some time > > > recently trying to > > > improve FTP reception by splicing from a TCP socket to a > > > file. I found that while splicing avoids copying packets to userland, > > > that gain is more than offset by a large increase in calls into the > > > storage stack.It's especially bad with TCP sockets because a typical > > > packet has, say,1460 bytes of data. Since splicing works on PIPE_BUFFERS > > > pages at a time, and packet pages are only about 35% utilized, each > > > cycle to userland I could only move 23 KiB of data at most. Some > > > similar effect may be in play in your case. > > > > > > > Agreed,increasing number of calls will offset the benefit. > > But what if: > > 1)We were to increase the PIPE_BUFFERS from '16' to '64' or 'some value'? > > What are the implications in the other parts of the kernel? > > This came up recently, one problem is that there a couple of kernel > functions having up to 3 stack-based arrays of dimension PIPE_BUFFER. So > the stack cost of increasing PIPE_BUFFERS can be quite high. I've > thought it might be nice if there was some mechanism for userland apps > to be able to request larger PIPE_BUFFERS values, but I haven't pursued > this line of thought to see if it's practical.
I still have patches pending for this, making the pipe buffer count settable form user space:
http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=24547ac4d97bebb58caf9ce58bd507a95c812a3f
Let me know if you want to give it a spin on a recent kernel, and I'll update it.
> > 2)There was a way to find out if the DMA-out/in from the initial buffer's that were passed are complete so that we are free to recycle them? Callback would be helpful.Obviously, the user-space-app will have to manage it's buffers but atleast we are guranteed that the buffers can be recycled(in other words no worrying about modifying in-flight data that is being DMA'd). > > It's a neat idea, but it would probably be much easier (and less > invasive) to try this sort of pipelining in userland using a ring buffer > or ping-pong approach. I'm actually in the middle of something like this > with FTP, where I will have a reader thread that puts data from the > network into a ring buffer, from which a writer thread moves it to a > file.
See vmsplice.c from the splice test tools:
http://brick.kernel.dk/snaps/splice-git-latest.tar.gz
-- Jens Axboe
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