Fix read example use io.CopyBuffer #42

nirs · 2024-11-03T20:32:34Z

Due to zero copy optimizations in os.File, using io.CopyBuffer() with
os.File and qcow2reader.Image ends as io.Copy() ignoring our buffer.
This is slow and a bad example for users of the library.

Fix the issue by hiding os.File ReadFrom method with a wrapper, forcing
io.CopyBuffer() to use our buffer.

This speeds up the copy significantly:

Reading qcow2 image is 3.7 times faster
Copying qcow2 image is 1.7 times faster
Reading qcow2 compressed image is 1.05 times faster

Based on the benchmarks, update the default buffer size in the read exxample
to 2 MiB.

Fixes: #41

AkihiroSuda · 2024-11-03T23:46:45Z

cmd/go-qcow2reader-example/read.go

@@ -29,7 +29,7 @@ func cmdRead(args []string) error {
 		flag.PrintDefaults()
 	}
 	fs.BoolVar(&debug, "debug", false, "enable printing debug messages")
-	fs.IntVar(&bufferSize, "buffer-size", 65536, "buffer size")
+	fs.IntVar(&bufferSize, "buffer-size", 512*1024, "buffer size")


This is a separate topic and should be another commit

Separated to another commit.

AkihiroSuda · 2024-11-03T23:47:21Z

cmd/go-qcow2reader-example/read.go

+
+// Writer forces os.File to write using our buffer by hiding it's ReadFrom
+// method. io.CopyBuffer is up to XX times faster with this wrrapper.
+type Writer struct {


Writer isn't a descriptive name.

What about:

Suggested change

type Writer struct {

type hideReadFrom struct {

AkihiroSuda · 2024-11-03T23:47:41Z

cmd/go-qcow2reader-example/read.go


 	return err
 }
+
+// Writer forces os.File to write using our buffer by hiding it's ReadFrom
+// method. io.CopyBuffer is up to XX times faster with this wrrapper.


Did you want to fulfill XX?

Maybe just add a link to this PR (#42) for the benchmark result and the context

I'll link the pr.

I updated the docs including a link to the pr.

AkihiroSuda · 2024-11-03T23:47:58Z

cmd/go-qcow2reader-example/read.go


 	return err
 }
+
+// Writer forces os.File to write using our buffer by hiding it's ReadFrom


it's → its ?

Not sure, too easy to get this wrong :-)

Right, should be "its"

Removed in current version.

Due to zero copy optimizations in os.File, using io.CopyBuffer() with os.File and qcow2reader.Image ends as io.Copy() ignoring our buffer. This is slow and a bad example for users of the library. Fix the issue by hiding os.File ReadFrom method with a wrapper, forcing io.CopyBuffer() to use our buffer. This speeds up the copy significantly: - Reading qcow2 image is 3.7 times faster - Copying qcow2 image is 1.7 times faster - Reading qcow2 compressed image is 1.05 times faster Making the example more complicated is not great but users need to know how to workaround the "optimizations" in io.CopyBuffer(). I think this should be fixed in the standard library later. Reading Ubuntu 24.04 image in qcow2 format: % hyperfine -w3 -r10 -L b 32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608 \ "./go-qcow2reader-example read -buffer-size {b} /var/tmp/images/test.qcow2 >/dev/null" Benchmark 1: ./go-qcow2reader-example read -buffer-size 32768 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 697.8 ms ± 13.0 ms [User: 107.2 ms, System: 591.8 ms] Range (min … max): 684.0 ms … 724.0 ms 10 runs Benchmark 2: ./go-qcow2reader-example read -buffer-size 65536 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 430.9 ms ± 4.7 ms [User: 75.3 ms, System: 356.5 ms] Range (min … max): 426.0 ms … 438.0 ms 10 runs Benchmark 3: ./go-qcow2reader-example read -buffer-size 131072 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 306.1 ms ± 3.7 ms [User: 62.2 ms, System: 244.2 ms] Range (min … max): 302.9 ms … 315.0 ms 10 runs Benchmark 4: ./go-qcow2reader-example read -buffer-size 262144 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 259.8 ms ± 5.8 ms [User: 55.6 ms, System: 204.2 ms] Range (min … max): 253.6 ms … 266.8 ms 10 runs Benchmark 5: ./go-qcow2reader-example read -buffer-size 524288 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 223.9 ms ± 2.9 ms [User: 49.5 ms, System: 170.9 ms] Range (min … max): 220.6 ms … 228.3 ms 10 runs Benchmark 6: ./go-qcow2reader-example read -buffer-size 1048576 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 201.1 ms ± 1.7 ms [User: 48.3 ms, System: 148.9 ms] Range (min … max): 198.0 ms … 204.2 ms 10 runs Benchmark 7: ./go-qcow2reader-example read -buffer-size 2097152 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 192.8 ms ± 3.2 ms [User: 47.5 ms, System: 140.4 ms] Range (min … max): 188.5 ms … 196.6 ms 10 runs Benchmark 8: ./go-qcow2reader-example read -buffer-size 4194304 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 189.3 ms ± 5.9 ms [User: 47.7 ms, System: 135.0 ms] Range (min … max): 183.5 ms … 201.5 ms 10 runs Benchmark 9: ./go-qcow2reader-example read -buffer-size 8388608 /var/tmp/images/test.qcow2 >/dev/null Time (mean ± σ): 191.4 ms ± 1.8 ms [User: 48.9 ms, System: 139.1 ms] Range (min … max): 189.8 ms … 195.5 ms 10 runs Summary ./go-qcow2reader-example read -buffer-size 4194304 /var/tmp/images/test.qcow2 >/dev/null ran 1.01 ± 0.03 times faster than ./go-qcow2reader-example read -buffer-size 8388608 /var/tmp/images/test.qcow2 >/dev/null 1.02 ± 0.04 times faster than ./go-qcow2reader-example read -buffer-size 2097152 /var/tmp/images/test.qcow2 >/dev/null 1.06 ± 0.03 times faster than ./go-qcow2reader-example read -buffer-size 1048576 /var/tmp/images/test.qcow2 >/dev/null 1.18 ± 0.04 times faster than ./go-qcow2reader-example read -buffer-size 524288 /var/tmp/images/test.qcow2 >/dev/null 1.37 ± 0.05 times faster than ./go-qcow2reader-example read -buffer-size 262144 /var/tmp/images/test.qcow2 >/dev/null 1.62 ± 0.05 times faster than ./go-qcow2reader-example read -buffer-size 131072 /var/tmp/images/test.qcow2 >/dev/null 2.28 ± 0.08 times faster than ./go-qcow2reader-example read -buffer-size 65536 /var/tmp/images/test.qcow2 >/dev/null 3.69 ± 0.13 times faster than ./go-qcow2reader-example read -buffer-size 32768 /var/tmp/images/test.qcow2 >/dev/null Copying Ubuntu 24.04 image in qcow2 format: % hyperfine -w3 -r10 -L b 32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608 \ "./go-qcow2reader-example read -buffer-size {b} /var/tmp/images/test.qcow2 >/tmp/tmp.img" Benchmark 1: ./go-qcow2reader-example read -buffer-size 32768 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 2.000 s ± 0.283 s [User: 0.126 s, System: 1.678 s] Range (min … max): 1.847 s … 2.792 s 10 runs Benchmark 2: ./go-qcow2reader-example read -buffer-size 65536 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 1.425 s ± 0.010 s [User: 0.096 s, System: 1.265 s] Range (min … max): 1.411 s … 1.447 s 10 runs Benchmark 3: ./go-qcow2reader-example read -buffer-size 131072 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 1.269 s ± 0.029 s [User: 0.077 s, System: 1.050 s] Range (min … max): 1.244 s … 1.333 s 10 runs Benchmark 4: ./go-qcow2reader-example read -buffer-size 262144 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 1.167 s ± 0.022 s [User: 0.058 s, System: 0.928 s] Range (min … max): 1.139 s … 1.206 s 10 runs Benchmark 5: ./go-qcow2reader-example read -buffer-size 524288 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 1.218 s ± 0.048 s [User: 0.048 s, System: 0.870 s] Range (min … max): 1.161 s … 1.289 s 10 runs Benchmark 6: ./go-qcow2reader-example read -buffer-size 1048576 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 1.210 s ± 0.046 s [User: 0.044 s, System: 0.855 s] Range (min … max): 1.148 s … 1.284 s 10 runs Benchmark 7: ./go-qcow2reader-example read -buffer-size 2097152 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 1.178 s ± 0.045 s [User: 0.041 s, System: 0.837 s] Range (min … max): 1.145 s … 1.296 s 10 runs Benchmark 8: ./go-qcow2reader-example read -buffer-size 4194304 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 1.170 s ± 0.034 s [User: 0.041 s, System: 0.861 s] Range (min … max): 1.140 s … 1.234 s 10 runs Benchmark 9: ./go-qcow2reader-example read -buffer-size 8388608 /var/tmp/images/test.qcow2 >/tmp/tmp.img Time (mean ± σ): 1.176 s ± 0.028 s [User: 0.043 s, System: 0.908 s] Range (min … max): 1.147 s … 1.221 s 10 runs Summary ./go-qcow2reader-example read -buffer-size 262144 /var/tmp/images/test.qcow2 >/tmp/tmp.img ran 1.00 ± 0.03 times faster than ./go-qcow2reader-example read -buffer-size 4194304 /var/tmp/images/test.qcow2 >/tmp/tmp.img 1.01 ± 0.03 times faster than ./go-qcow2reader-example read -buffer-size 8388608 /var/tmp/images/test.qcow2 >/tmp/tmp.img 1.01 ± 0.04 times faster than ./go-qcow2reader-example read -buffer-size 2097152 /var/tmp/images/test.qcow2 >/tmp/tmp.img 1.04 ± 0.04 times faster than ./go-qcow2reader-example read -buffer-size 1048576 /var/tmp/images/test.qcow2 >/tmp/tmp.img 1.04 ± 0.05 times faster than ./go-qcow2reader-example read -buffer-size 524288 /var/tmp/images/test.qcow2 >/tmp/tmp.img 1.09 ± 0.03 times faster than ./go-qcow2reader-example read -buffer-size 131072 /var/tmp/images/test.qcow2 >/tmp/tmp.img 1.22 ± 0.02 times faster than ./go-qcow2reader-example read -buffer-size 65536 /var/tmp/images/test.qcow2 >/tmp/tmp.img 1.71 ± 0.24 times faster than ./go-qcow2reader-example read -buffer-size 32768 /var/tmp/images/test.qcow2 >/tmp/tmp.img Fixes: lima-vm#41 Signed-off-by: Nir Soffer <[email protected]>

Based on the benchmarks, 2 MiB buffer gives good results for reading and copying an image with the read example. Signed-off-by: Nir Soffer <[email protected]>

AkihiroSuda

Thanks

AkihiroSuda reviewed Nov 3, 2024

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nirs added 2 commits November 4, 2024 05:46

Update default buffer size for read example

9fef7ca

Based on the benchmarks, 2 MiB buffer gives good results for reading and copying an image with the read example. Signed-off-by: Nir Soffer <[email protected]>

nirs force-pushed the fix-read-example branch from ee2b114 to 9fef7ca Compare November 4, 2024 13:27

nirs requested a review from AkihiroSuda November 4, 2024 13:35

AkihiroSuda approved these changes Nov 4, 2024

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AkihiroSuda merged commit a04a2a1 into lima-vm:master Nov 4, 2024
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nirs deleted the fix-read-example branch November 4, 2024 14:00

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Fix read example use io.CopyBuffer #42

Fix read example use io.CopyBuffer #42

nirs commented Nov 3, 2024 •

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Fix read example use io.CopyBuffer #42

Fix read example use io.CopyBuffer #42

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nirs commented Nov 3, 2024 •

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