[Xapian-discuss] redundant disk access in xapian-tcpsrv

[Rocco Caputo]

Hi.

SocketServer::run_getdocument() triggers two record_table->get_record 
(did) calls in the flint backend.  One when the database's  
open_document() is called in non-lazy mode, and another when  
run_getdocument() calls doc->get_data().

I noticed this while looking through an strace of xapian-tcpsrv to  
determine the reason it's slower at uncached fetches than my stand- 
alone Perl client.  To be fair, my stand-alone test client also opens  
the database in non-lazy mode, so it suffers from the same  
inefficiency that xapian-tcpsrv seems to.  Also, calling open_document 
() in lazy mode removes the redundant lseek() and read() calls, but  
this doesn't improve performance in my situation.

non-lazy open_document() strace:

lseek(28, 6414336, SEEK_SET)            = 6414336
read(28, "\0\0\3\306\1\6\20\6\26\3\177\37\363\37\346\f\317\37\331"...
lseek(28, 363642880, SEEK_SET)          = 363642880
read(28, "\0\0\3\232\0\5N\v\245\0\35\36\365\6\305\33\"\31\355\27"...
lseek(28, 363659264, SEEK_SET)          = 363659264
read(28, "\0\0\3\232\0\10@\25\331\0\31\10Y\27<\25\367\24\233\23\36"...
lseek(28, 363642880, SEEK_SET)          = 363642880
read(28, "\0\0\3\232\0\5N\v\245\0\35\36\365\6\305\33\"\31\355\27"...
lseek(28, 363659264, SEEK_SET)          = 363659264
read(28, "\0\0\3\232\0\10@\25\331\0\31\10Y\27<\25\367\24\233\23\36"...
gettimeofday({1151443453, 519154}, NULL) = 0
gettimeofday({1151443453, 519212}, NULL) = 0
select(55, NULL, [54], [54], {14, 999942}) = 1 (left {15, 0})
write(54, "O!)!*!\'!$!$!$!,!.6xxxxxxxx!D|!DN"..., 3684) = 3684
gettimeofday({1151443453, 519416}, NULL) = 0
gettimeofday({1151443453, 519472}, NULL) = 0
select(55, NULL, [54], [54], {14, 999944}) = 1 (left {15, 0})
write(54, "Z\n", 2)                     = 2

lazy strace:

lseek(28, 6414336, SEEK_SET)            = 6414336
read(28, "\0\0\3\306\1\6\20\6\26\3\177\37\363\37\346\f\317\37\331"...
lseek(28, 363642880, SEEK_SET)          = 363642880
read(28, "\0\0\3\232\0\5N\v\245\0\35\36\365\6\305\33\"\31\355\27"...
lseek(28, 363659264, SEEK_SET)          = 363659264
read(28, "\0\0\3\232\0\10@\25\331\0\31\10Y\27<\25\367\24\233\23\36"...
gettimeofday({1151450138, 328166}, NULL) = 0
gettimeofday({1151450138, 328222}, NULL) = 0
select(55, NULL, [54], [54], {14, 999944}) = 1 (left {15, 0})
write(54, "O!)!*!\'!$!$!$!,!.6xxxxxxxx!D|!DN"..., 3684) = 3684
gettimeofday({1151450138, 328426}, NULL) = 0
gettimeofday({1151450138, 328484}, NULL) = 0
select(55, NULL, [54], [54], {14, 999942}) = 1 (left {15, 0})
write(54, "Z\n", 2)                     = 2

-- 
Rocco Caputo - rcaputo at pobox.com