On 12/05/2012 03:11 PM, Keith Chew wrote:
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.
A bunch of sequential writes to the wal file, totaling somewhere
around 4KB. This should be fast. But in the middle you have a 1
second delay:
> [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
If you use -T instead of -t with strace, it reports the amount
of wall-clock time spent in each system call.
Dan.
[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
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