Mkfile(8) is severely syscall limited on OS X

http://blog.metaobject.com/2017/02/mkfile8-is-severely-syscall-limited-on.html

by mpweiher • 9 years ago

102 78 9 years ago

blog.metaobject.com

im3w1l• 9 years ago

Oh wow, I had no idea that write buffers needed to be on the order of a megabyte on modern systems.

mpweiher8• 9 years ago

Neither did I! And in fact it took almost 2 days to question/overcome my assumptions.

"It's not what you don't know that kills you. It is what you know for sure that ain't true" -- Mark Twain

microtonal2• 9 years ago

Interesting... You never dd data to a USB3 disk or SSD? The default buffer size for dd on OS X is also 512 bytes. I've been using `bs=1m` on OS X for ages.

fulafel• 9 years ago

It's just OS X being slow, Linux gets close to peak with a 16 kB buffer: http://blog.tdg5.com/tuning-dd-block-size/

mpweiher8• 9 years ago

Maybe I am reading it wrong, but I am not getting the 16KB optimal buffer size from that article. The summary at the end is as follows:

'In the above example it can be seen that an input block size of 128K is optimal for my particular setup.'

However, the SSD seems to be significantly slower, so the relative overhead of the syscall is going to be respectively less:

'Reading from /dev/zero and writing out to a SSD with a more optimal block size of 256K yields a throughput of 280 MB/s.'

I was getting 250MB/s with the 512 byte buffer size, although the test in the article is doing two syscalls (one read(), one write()) whereas mkfile is just doing one.

At 256KB buffer size I was already getting around 1.8GB/second writing to SSD.

feld2• 9 years ago

Or is it another case of Linux taking shortcuts and sacrificing safety for speed?

alain94040• 9 years ago

Reading from /dev/zero and writing out to RAM with a more optimal block size of 256K yields a throughput of 1,536 MB/s

Actually, in this article, Linux writing to RAM is slower than MacOS writing to SSD. Hard to conclude that "it's just OS X being slow". I suspect the hardware used is wildly different from the one in the original article.

ivoras• 9 years ago

Just saw the performance of this "consumer" device and remembered this thread: http://www.anandtech.com/show/11104/the-patriot-hellfire-m2-... . This SSD can write over a GB/s and read nearly 3 GB/s. We've come a long way since 32 KiB buffers were a sane choice.

amelius• 9 years ago

So is this any different on Linux? Why is OSX in the title?

Are syscalls more expensive on OSX?

mpweiher8• 9 years ago

Good point, the reason is simple: I didn't check on Linux (and have updated the post to reflect this). I am guessing it would be similar, but don't have any data.

throwawayish3• 9 years ago

It is.

akandiah• 9 years ago

So is this any different on Linux? Why is OSX in the title?

mkfile is not available on Linux. The equivalent utility is xfs_mkfile or fallocate.

throwawayish3• 9 years ago

No, mkfile creates a file with written extents (no sparse or uninitialized areas), while fallocate will (with ext4/xfs/...) only map some extents which are marked uninitialized. So while mkfile 10G would have to write ~10 GB of data, fallocate 10G is nearly instanteneous, because it most likely just allocated a single extent.

xfs_mkfile could do what mkfile does, the description isn't conclusive enough.

mschuster912• 9 years ago

I guess it is the same. Try doing a dd if=/dev/sdX of=/dev/null or dd if=/dev/zero of=/dev/sdX (be warned, the latter wipes the disk!).

Then, experiment with various buffer sizes (bs=1k, 10k, 100k, 1M) - I personally use 1M with dd.

Be warned on OS X you have to use /dev/rdiskX instead of /dev/diskX, as the latter is a buffered version that usually is slower.

masklinn3• 9 years ago

You know, you can dd to a file to avoid destroying a drive. And you can provide a `count` to write fixed-size output files, dd will write bs×count bytes (technically it writes ibs×count, and bs sets both ibs and obs).

    dd if=/dev/zero of=foo bs=1k count=1m

would write 1GB in 1k blocks, and

    dd if=/dev/zero of=foo bs=1m count=1k

would write 1GB in 1m blocks.

Svip• 9 years ago

I think the main difference is that `mkfile' doesn't exist on Linux? As far as I can gather, it's a Sun OS utility, that BSD has and therefore OS X.

I've read that some Linux distributions have `mkfile', but it's just a script wrapper around `dd'.

geocar2• 9 years ago

It's interesting that if you google "how do i create a large file on osx" you get guidance for this command[1].

But yes, syscalls are more expensive on OSX.

[1] http://stackoverflow.com/questions/26796729/quickly-create-a...

bogomipz2• 9 years ago

"But yes, syscalls are more expensive on OSX."

Can you elaborate on this?

The XNU implements system calls in the same way a BSD system does. If you are talking about the Mach aspect of OS X, the BSD part can call down to the the Mach directly without using Mach messages. XNU is not a traditional microkernel even though Mach is in there.

pedrocr• 9 years ago

The graph could use an X axis label. I'm assuming it's "Buffer size in kB". It would also be nice to include the datapoint you started with (250MB/s from a 512byte buffer).

masklinn3• 9 years ago

That's actually written below the graph, rather than as a graph label

X-Axis is buffer size in KB. The original 512 byte size isn't on there because it would be 0.5KB or the entire axis would need to be bytes, which would also be awkward at the larger sizes. Also note that the X-Axis is logarithmic.

mpweiher8• 9 years ago

Sorry, I should have mentioned that I added the comment after the parent's very valid point.

DougBTX• 9 years ago

I don't understand that reasoning, 0.5 on a logarithmic base 2 scale would be the next step below 1 on the x axis (0.5, 1, 2, 4, 8), and the y axis already goes to zero anyway.

Retr0spectrum• 9 years ago

Furthermore, the X and Y axis should really have been the other way around.

mpweiher8• 9 years ago

Though I thought it was fairly obvious from the text this is still a good point. Added as comment underneath.

Matt3o12_• 9 years ago

This still begs the question I've had since day one when learning about buffers: what the heck is the recommended buffer size. I've seen a lot of old code that use extremely small buffer size as well as some recent code that use extremely high buffer sizes (up to 10MB).

What is a good sweet spot that runs well on older hardware (less then 10 years old) and new hardware? And should network buffers be bigger or smaller then disk buffers?

throwawayish3• 9 years ago

Network buffers are a completely different animal, also harder to optimize...

Disk IO buffers, it's easy nowadays, just use something like a MB, which is just fine for almost any application, and doesn't stack up to much memory use (unless you're writing many files concurrently, which can bring it's own problems as well)

yjftsjthsd-h• 9 years ago

That actually does sound about right, but do you know how to derive that? I'm curious how to figure it out.

RyanZAG• 9 years ago

Judging by the post here, 512kb buffers seems like a good bet?

mpweiher8• 9 years ago

For a an SSD with 2GB/s throughput and the syscall overhead of OS X: yes. Go higher if you want some headroom for the next generation of faster SSDs.

gens3• 9 years ago

Just do a test with dd or some small piece of code. Why rely on heresay.

dom03• 9 years ago

And that's why sane people will tell you to not use something like st_blksize as the IO size, because that's far to small. Many tools use 32 KiB, but that can be limiting even on Linux and needlessly uses more CPU (especially if FUSE is involved -- reads are not coalesced at the VFS layer! Read and write merging is done at the block device layer by the IO scheduler there!). Something like 512k-1M is a sane default these days.

gens3• 9 years ago

Maybe i'm misunderstanding you, but.. Before every one a "echo 3 > /proc/sys/vm/drop_caches" is executed. Processor is in "performance" mode.

dd if=./big_file.file of=/dev/null bs=64K

535020540 bytes (535 MB, 510 MiB) copied, 8.70128 s, 61.5 MB/s

dd if=./big_file.file of=/dev/null bs=4K

535020540 bytes (535 MB, 510 MiB) copied, 8.36283 s, 64.0 MB/s

dd if=./big_file.file of=/dev/null bs=1K

535020540 bytes (535 MB, 510 MiB) copied, 8.82508 s, 60.6 MB/s

Only thing gone up is the cpu usage (same is negligible at ~60MB/s).

dd if=/dev/zero of=/dev/null bs=1K count=1024K

1073741824 bytes (1.1 GB, 1.0 GiB) copied, 1.06113 s, 1.0 GB/s

dd if=/dev/zero of=/dev/null bs=4K count=256K

1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.343595 s, 3.1 GB/s

dd if=/dev/zero of=/dev/null bs=32K count=32K

1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.160191 s, 6.7 GB/s

brobinson• 9 years ago

Don't forget "oflag=direct"

dom03• 9 years ago

At just 60 MB/s it doesn't matter much (even 512 byte blocks would just be a little more than 100k syscalls / second). At a couple hundred MB/s it begins to matter, and with faster NVMe SSDs it's a legit issue.

This becomes a bigger issue if it's not a kernel-based, local FS, but something that does more work, I mentioned FUSE above. Since no read merging happens, small reads like 4k or 32k start to hurt earlier, especially if the FS is written in an interpreter. Some of these also mishandled small reads, incurring an additional penalty (I know because I fixed that in an implementation).

cbsmith4• 9 years ago

The whole thing is kind of odd. Normally I'd want to leave filling the file to filesystem...

JelteF• 9 years ago

Interesting post, but this xkcd is really relevant here: https://xkcd.com/833/

mkj2• 9 years ago

It's a command, not a syscall!

quinnftw• 9 years ago

Yes but mkfile uses syscalls to create and write to the file. With a smaller buffer more write syscalls are required to populate the file.

Sharlin• 9 years ago

The article doesn't make such a claim as far as I can see.

mkj2• 9 years ago

You're right, I misread. Sorry for the noise.

drewg123• 9 years ago

" I did not check on other operating systems, but my guess is that the results would be similar."

Actually, no. Irrespective of mkfile being mostly MacOSX specific, most sycalls on MacOSX are just plain slow as compared to Linux (or FreeBSD). I think this is partially an artifact of performance not being a major metric for most system calls on MacOSX. The systems calls where performance is a critical metric (like timekeeping) use a kernel / user shared page interface to avoid the syscall entirely in the vast majority of the cases.

I recall doing benchmarking roughly 10 years ago to find the best interface to communicate with a mostly in an OS-bypass HPC driver. MacOSX ioctls were some multipler more expensive than Linux, but the native IOKit Mach IPC was even more expensive than that. Sigh. There was a similar story for sockets in non-OS bypass mode, where simply writing on a socket was far more expensive than Linux.

Somebody needs to resurrect lmbench & do a comparison of the various x86 kernels available these days. Maybe that would shame Apple into focusing on performance.

Drew

cbsmith4• 9 years ago

I'm pretty sure that even at their slowest, syscalls aren't going to match IO overhead. It's not like the mkfile is CPU bound.

mpweiher8• 9 years ago

The point of the article is that I was also "pretty sure" about exactly that, and advances in hardware performance meant I was wrong...

2GB/s is pretty damn fast, 0.5 nanoseconds per byte.

3wisemen• 9 years ago

Using mkfile compiled for Linux on a tmpfs gets me 1550 MiB/s.

raverbashing• 9 years ago

Well, there's this option (which looks like it would be the way to go for its purpose):

-n Create an empty filename. The size is noted, but disk blocks aren't allocated until data is written to them

Also the description on the man file says: mkfile creates one or more files that are suitable for use as NFS-mounted swap areas. The sticky bit is set, and the file is padded with zeroes by default

grigjd3• 9 years ago

That's missing the point. The author wanted to test the write speed and found that mkfile was not a good tool to test with.

rollthehard6• 9 years ago

Some years ago I picked up the habit from a predecessor of testing such things with dd instead, that way you can experiment with the effect of different block sizes, so like -

dd of=/dev/zero of=/ddtest.out myfile bs=64k count=65536

mcguire• 9 years ago

Does anyone know offhand the architecture of the Mach/BSD hybrid kernel in this case? It sounds suspiciously like a problem with the IBM Microkernel back in about 1992, and with the OSF's mkLinux, which is apparently a predecessor of Mac OS.

Specifically, do the syscalls writing the file go through multiple protection domains?

https://maniagnosis.crsr.net/2011/07/this-is-comment-i-made-...

Someone• 9 years ago

"the OSF's mkLinux, which is apparently a predecessor of Mac OS."

Apple worked on MkLinux, but it isn't technically a predecessor to Mac OS X. The two do not share a single line of code; if they did, Apple would have to license Mac OS under the GPL.

XNU, the Mac OS kernel (https://en.wikipedia.org/wiki/XNU) isn't a real microkernel; the functionality of a microkernel is there, but quite a bit of code was added that, in a true microkernel, would live in userspace.

gwu78• 9 years ago

While HN is discussing XNU sycalls, anyone know why the most fundamental of syscalls, execve, according to NeXT/Apple's post-UNIX wisdom needs to have an extra "char *apple[]" argument vector?

Not to imply that it is "hidden" but I am curious if any HN users know about this and understand its purpose?

seba_dos1• 9 years ago

https://unixjunkie.blogspot.com/2006/02/char-apple-argument-...

msbarnett• 9 years ago

When in doubt, consult the source: https://github.com/opensource-apple/xnu/blob/53c5e2e62fc4182...

The upshot is that it looks like it constructs some non-env variable environment data on the program's stack after the posix environment.

The absolute path of the executable, followed by preemption free zone addresses, entropy, a configuration setting for malloc allocation strategy, and the address of the main thread's stack afaict.

pixelbeat__• 9 years ago

The GNU coreutils default to 128KiB buffers as per the test script at

https://github.com/coreutils/coreutils/blob/master/src/ioblk...

snorrah• 9 years ago

Is this something that would be affected by file system defaults? I see it writes in 512 byte chunks by default, when MacOS moves to APFS, is this something that might be altered as a result and thus get a 'free' speedup?

Mkfile(8) is severely syscall limited on OS X

Related Stories