Messages in this thread Patch in this message | | | Subject | Re: splice/vmsplice performance test results | From | "Jim Schutt" <> | Date | Tue, 21 Nov 2006 12:17:49 -0700 |
| |
On Tue, 2006-11-21 at 14:54 +0100, Jens Axboe wrote: > On Mon, Nov 20 2006, Jim Schutt wrote: > > On Mon, 2006-11-20 at 09:24 +0100, Jens Axboe wrote: > > > On Mon, Nov 20 2006, Jens Axboe wrote: > > > > On Fri, Nov 17 2006, Jim Schutt wrote: > > > > > On Thu, 2006-11-16 at 21:25 +0100, Jens Axboe wrote: > > > > > > On Thu, Nov 16 2006, Jim Schutt wrote: > > > > > > > Hi, > > > > > > > > > > > > > > > > > > > My test program can do one of the following: > > > > > > > > > > > > > > send data: > > > > > > > A) read() from file into buffer, write() buffer into socket > > > > > > > B) mmap() section of file, write() that into socket, munmap() > > > > > > > C) splice() from file to pipe, splice() from pipe to socket > > > > > > > > > > > > > > receive data: > > > > > > > 1) read() from socket into buffer, write() buffer into file > > > > > > > 2) ftruncate() to extend file, mmap() new extent, read() > > > > > > > from socket into new extent, munmap() > > > > > > > 3) read() from socket into buffer, vmsplice() buffer to > > > > > > > pipe, splice() pipe to file (using the double-buffer trick) > > > > > > > > > > > > > > Here's the results, using: > > > > > > > - 64 KiB buffer, mmap extent, or splice > > > > > > > - 1 MiB TCP window > > > > > > > - 16 GiB data sent across network > > > > > > > > > > > > > > A) from /dev/zero -> 1) to /dev/null : 857 MB/s (6.86 Gb/s) > > > > > > > > > > > > > > A) from file -> 1) to /dev/null : 472 MB/s (3.77 Gb/s) > > > > > > > B) from file -> 1) to /dev/null : 366 MB/s (2.93 Gb/s) > > > > > > > C) from file -> 1) to /dev/null : 854 MB/s (6.83 Gb/s) > > > > > > > > > > > > > > A) from /dev/zero -> 1) to file : 375 MB/s (3.00 Gb/s) > > > > > > > A) from /dev/zero -> 2) to file : 150 MB/s (1.20 Gb/s) > > > > > > > A) from /dev/zero -> 3) to file : 286 MB/s (2.29 Gb/s) > > > > > > > > > > > > > > I had (naively) hoped the read/vmsplice/splice combination would > > > > > > > run at the same speed I can write a file, i.e. at about 450 MB/s > > > > > > > on my setup. Do any of my numbers seem bogus, so I should look > > > > > > > harder at my test program? > > > > > > > > > > > > Could be read-ahead playing in here, I'd have to take a closer look at > > > > > > the generated io patterns to say more about that. Any chance you can > > > > > > capture iostat or blktrace info for such a run to compare that goes to > > > > > > the disk? > > > > > > > > > > I've attached a file with iostat and vmstat results for the case > > > > > where I read from a socket and write a file, vs. the case where I > > > > > read from a socket and use vmsplice/splice to write the file. > > > > > (Sorry it's not inline - my mailer locks up when I try to > > > > > include the file.) > > > > > > > > > > Would you still like blktrace info for these two cases? > > > > > > > > No, I think the iostat data is fine, I don't think the blktrace info > > > > would give me any more insight on this problem. I'll set up a test to > > > > reproduce it here, looks like the write out path could be optimized some > > > > more. > > > > Great, let me know if you need testing from me. > > I found some suboptimal behaviour in your test app - you don't check for > short reads and splice would really like things to be aligned for the > best performance. I did some testing with the original app here, and I > get 114.769MB/s for read-from-socket -> write-to-file and 109.878MB/s > for read-from-socket -> vmsplice-splice-to-file. If I fix up the read to > always get the full buffer size before doing the vmsplice+splice, the > performance is up to the same as the read/write.
Sorry - I had assumed my network was so much faster than my disk subsystem I'd never get a short read from a socket except at the end of the transfer. Pretty silly of me, in hindsight.
I can see now how even one short read early would screw up the alignment for splicing into a file for the rest of the transfer, right?
Here's some new results:
Run w/check for short read on socket in vmsplice case: - /dev/zero -> /dev/null w/ socket read + file write: 1130 MB/s (Man, my network is running fast today. I don't know why.) - /dev/zero -> /dev/null w/ socket read + vmsplice/splice: 1028 MB/s - /dev/zero -> file w/ socket read + vmsplice/splice: 336 MB/s
Rerun w/original: - /dev/zero -> /dev/null w/ socket read + vmsplice/splice: 1026 MB/s - /dev/zero -> file w/ socket read + vmsplice/splice: 285 MB/s - /dev/zero -> file w/ socket read + file write: 382 MB/s
So I was losing 50 MB/s due to short reads on the socket screwing up the alignment for splice. Sorry to waste your time on that.
But, it looks like socket-read + file-write is still ~50 MB/s faster than socket-read + vmsplice/splice (assuming I didn't screw up my short read fix - see patch below). I assume that's still unexpected?
> > Since it's doing buffered writes, the results do vary a lot though (as > you also indicated). A raw /dev/zero -> /dev/null is 3 times faster with > vmsplice/splice. >
Hmmm. Is it worth me trying to do some sort of kernel profiling to see if there is anything unexpected with my setup? If so, do you have a preference as to what I would use?
Here's how I fixed my app to fix up (I think) short reads. Maybe I missed your point?
diff --git a/src/dnd.c b/src/dnd.c index 01bd7b8..aa70102 100644 --- a/src/dnd.c +++ b/src/dnd.c @@ -773,18 +773,26 @@ uint64_t vmsplice_recv(const struct opti again: i = (i + 1) & 1; iov.iov_base = opts->buf + i * opts->buf_size; + l = 0; again2: - l = read(sd, iov.iov_base, opts->buf_size); - if (l < 0) { + m = read(sd, iov.iov_base + l, opts->buf_size - l); + if (m < 0) { if (errno == EINTR) goto again2; perror("Read"); exit(EXIT_FAILURE); } - if (l == 0) { - fdatasync(fd); - return bytes; + if (m == 0) { + if (l == 0) { + fdatasync(fd); + return bytes; + } + } + else { + l += m; + if (l != opts->buf_size) + goto again2; } while (l) {
- 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/
| |