Hmmm...looking at that strace sequencing is there some reason those 24-byte and 1024-byte writes can't be combined? The 1024-byte write is occurring at the end boundary of the 24-byte.
That would cut the i/o ops in half and might be a noticeable improvement. A memory copy would be a lot faster than 2 i/o requests. And since disk page size is always a power of 2 would the 1024 buffer be better off as 1000 to align the page i/o better? Michael D. Black Senior Scientist Advanced Analytics Directorate Advanced GEOINT Solutions Operating Unit Northrop Grumman Information Systems ________________________________________ From: sqlite-users-boun...@sqlite.org [sqlite-users-boun...@sqlite.org] on behalf of Keith Chew [keith.c...@gmail.com] Sent: Wednesday, December 05, 2012 2:11 AM To: General Discussion of SQLite Database Subject: EXT :Re: [sqlite] Tracing latencies Hi Dan On Wed, Dec 5, 2012 at 6:38 PM, Dan Kennedy <danielk1...@gmail.com> wrote: > If it's not fsync() then IO delays are normally caused by read(). > You could try [strace -T -eread ...] to check. > > Are SELECT statements fast on the same database? How large is the > database compared to the machines memory? Ah, do you think the read on the OS level is starving the writes? There are not many DB selects from the app, but there are other read IO activities happening in the background. Still it doesn't make sense, because WAL mode ensures from sqlite's point of view, the reader will not be blocked by the writer. So, sqlite is subjected to similar levels of read IO activity as Mysql (because there are very few DB selects). The tables are small, around 20MB in total, compared to 2GB of memory available. I have done more investigation. Using strace, I waited to capture an insert/update with a long latency, and caught one that is 2s long, below is the strace to the WAL file. We can see that it is very bursty, all the seeks and writes span close to over 1s. I recall doing a strace on Mysql a long time ago, and each insert only does a single seek and write. Maybe it is sqlite's file format that requires it to seek/write multiple places causing the slowness? I am only guessing here, as I do not know how to interpret the strace logs below. [pid 4015] 21:01:53.634099 _llseek(98, 499928, <unfinished ...> [pid 4015] 21:01:53.634245 write(98, "\0\0\0\4\0\0HU^=\226\213\23\10<\247+\214\332\260\314Wf ", 24 <unfinished ...> [pid 4015] 21:01:53.634546 _llseek(98, 499952, <unfinished ...> [pid 4015] 21:01:53.634712 write(98, "\r\0\0\0\1\0033\0\0033\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1024 <unfinished ...> [pid 4015] 21:01:53.740378 _llseek(98, 500976, [500976], SEEK_SET) = 0 [pid 4015] 21:01:53.740449 write(98, "\0\0\0\3\0\0\0\0^=\226\213\23\10<\247\327\201>\32\227\323\f8", 24) = 24 [pid 4015] 21:01:53.740521 _llseek(98, 501000, [501000], SEEK_SET) = 0 [pid 4015] 21:01:53.740566 write(98, "\r\3\221\0\t\3G\1\3\243\3\261\3\352\3\325\3\304\3G\3\177\3m\3\\\0\0\0\0\0\0"..., 1024) = 1024 [pid 4015] 21:01:53.740632 _llseek(98, 502024, [502024], SEEK_SET) = 0 [pid 4015] 21:01:53.740677 write(98, "\0\0\37X\0\0\0\0^=\226\213\23\10<\247dL\17\316\32\30\301\237", 24) = 24 [pid 4015] 21:01:53.740736 _llseek(98, 502048, [502048], SEEK_SET) = 0 [pid 4015] 21:01:53.740781 write(98, "\r\0\0\0\5\1*\0\3o\2\336\2L\1\273\1*\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1024) = 1024 [pid 4015] 21:01:53.740844 _llseek(98, 503072, [503072], SEEK_SET) = 0 [pid 4015] 21:01:53.740889 write(98, "\0\0HR\0\0HU^=\226\213\23\10<\247\276\32g\304j\372Q.", 24) = 24 [pid 4015] 21:01:53.740945 _llseek(98, 503096, [503096], SEEK_SET) = 0 [pid 4015] 21:01:53.740989 write(98, "\n\0\0\0\25\1\264\0\2@\2\\\2x\2\224\2\260\2\314\2\350\3\4\3 \3<\3X\3t"..., 1024) = 1024 [pid 4015] 21:01:54.636566 _llseek(98, 504120, [504120], SEEK_SET) = 0 [pid 4015] 21:01:54.636636 write(98, "\0\0\0\4\0\0HU^=\226\213\23\10<\247=]`\3700\351\226n", 24) = 24 [pid 4015] 21:01:54.636812 _llseek(98, 504144, [504144], SEEK_SET) = 0 [pid 4015] 21:01:54.636860 write(98, "\r\0\0\0\1\0033\0\0033\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1024 <unfinished ...> [pid 6744] 21:01:54.676590 close(98) = 0 Regards Keith _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users