C - read vs mmap

Read and mmap are both fundamentally important system calls, used to access bytes in files. Read uses the standard file descriptor to access files while mmap maps files to RAM. The purpose of this experiment was to compare performance differences with read when compared to mmap.

Approach

1. Randomly generate file content for different file sizes.
2. Calculate file histogram and measure elapsed time - 10 times
3. Write elapsed time, average time and standard deviation to a file.

These tests were conducted on a 5.4.0-77-generic Linux Mint 20.1 system utilizing an Intel Core i7-9750H @ 2.60GHz × 6 with 9.3.0 gcc version.

Getting started

Clone the project or download it as a zip and then unzip it.

Conduct standard test

Run

bash script.sh -h

and then use the appropriate flag.

Conduct custom test

Run

make
./main -h

and then use the appropriate flag.

Note: you can firstly use bash script.sh -c to generate test files.

Results

Buffer sizes are in bytes, time is in milliseconds.

File size: 512B

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	0.0064	0.0049	0.0053	0.0067	0.0099	0.0105
Standard deviation	0.0025	0.0022	0.0028	0.0028	0.0024	0.0048

Better method: read(1024), by 53.33%

---

File size: 2kB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	0.0096	0.0078	0.0062	0.0084	0.0115	0.0139
Standard deviation	0.0023	0.0022	0.0024	0.0022	0.0023	0.0067

Better method: read(4096), by 55.40%

---

File size: 8kB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	0.0251	0.0189	0.0135	0.0152	0.0169	0.0362
Standard deviation	0.0036	0.0033	0.0034	0.0038	0.0033	0.0209

Better method: read(4096), by 62.71%

---

File size: 32kB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	0.0859	0.0608	0.0489	0.0400	0.0410	0.0616
Standard deviation	0.0061	0.0058	0.0177	0.0068	0.0057	0.0174

Better method: read(8192), by 35.06%

---

File size: 128kB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	0.3414	0.2343	0.1532	0.1570	0.2329	0.1879
Standard deviation	0.0262	0.0136	0.0140	0.0407	0.0651	0.0307

Better method: read(4096), by 18.47%

---

File size: 512kB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	1.3204	0.9052	0.5982	0.5411	0.5155	0.6608
Standard deviation	0.0385	0.0525	0.0474	0.0433	0.0410	0.0810

Better method: read(16384), by 21.99%

---

File size: 1MB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	2.7041	1.8161	1.1783	1.0879	1.1230	1.9790
Standard deviation	0.1402	0.0729	0.0818	0.0866	0.0858	0.1878

Better method: read(8192), by 45.03%

---

File size: 8MB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	22.9465	15.2235	11.0226	9.5879	8.9895	8.8639
Standard deviation	2.3845	0.7469	1.7879	1.0304	0.8315	4.8685

Better method: mmap, by 1.4%

---

File size: 32MB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	88.1362	58.6678	39.9459	35.9764	33.9954	27.9418
Standard deviation	6.7785	2.6128	2.6215	2.5052	2.8756	7.7543

Better method: mmap, by 17.81%

---

File size: 256MB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	682.8666	466.3594	313.6546	285.8377	271.1903	216.3714
Standard deviation	21.2080	19.3882	19.5440	18.7643	20.0427	24.3847

Better method: mmap, by 20.21%

---

File size: 1GB

	read(512)	read(1024)	read(4096)	read(8192)	read(16384)	mmap
Average	2780.2674	1915.0757	1300.4177	1181.8023	1065.5766	845.8249
Standard deviation	54.3265	67.8010	74.5956	92.7043	75.9196	95.8478

Better method: mmap, by 20.62%

Conclusion

For smaller files, the read method is faster when compared to the mmap method. That changes with large files - mmap is faster for around 15%.

For smaller files, the most appropriate buffer size would be 4096B or 8192B, which is the same as a page size on the computer used in the experiment. The page size can be found by running getconf PAGE_SIZE.

For larger files, the most suitable buffer size would be 16kB.