slowdown in notmuch perf suite with xapian 1.3.5

[David Bremner]

I hadn't noticed any interactive slowdown, but when I got around to
running the notmuch performance suite, there seems to be some noticable
slowdown with the glass backend (default in Xapian 1.3.5) compared to
chert (using xapian 1.2.22)

These tests are on an older i7 with 12G of RAM and an SSD. I'm
reasonable confident they are CPU bound. One curious thing is the
increase in system time in the glass case. It also looks like the glass
backend is doing a lot more I/O, which could be related.

The current notmuch performance corpus has about 200k documents,
totalling about 3.5G. Unfortunately each number here represents only a
single run. I did rerun the tests with the glass backend, and the
variation was reasonably small.

Chert
=====

T00-new.sh: Testing notmuch new                         [0.4 large]
			Wall(s)	Usr(s)	Sys(s)	Res(K)	In/Out(512B)
  Initial notmuch new   669.06	639.78	21.42	323684	3576/9360440
  notmuch new #2        0.46	0.00	0.00	8240	3568/200
  notmuch new #3        0.01	0.00	0.00	7916	0/8
  notmuch new #4        0.01	0.01	0.00	8008	0/8
  notmuch new #5        0.01	0.00	0.00	8040	0/8
  notmuch new #6        0.01	0.00	0.00	8040	0/8

T01-dump-restore.sh: Testing dump and restore           [0.4 large]
			Wall(s)	Usr(s)	Sys(s)	Res(K)	In/Out(512B)
  load nmbug tags       5.85	2.64	0.10	11280	1376/40496
  dump *                7.45	6.51	0.94	25272	104/27928
  restore *             7.55	7.15	0.39	8180	0/0

T02-tag.sh: Testing tagging                             [0.4 large]
			Wall(s)	Usr(s)	Sys(s)	Res(K)	In/Out(512B)
  tag * +new_tag        200.13	183.01	7.08	38628	264/1664552
  tag * +existing_tag   0.00	0.00	0.00	8356	0/0
  tag * -existing_tag   153.47	145.00	4.02	34928	0/1626320
  tag * -missing_tag    0.00	0.00	0.00	8252	0/0

Glass
=====

T00-new.sh: Testing notmuch new                         [0.4 large]
			Wall(s)	Usr(s)	Sys(s)	Res(K)	In/Out(512B)
  Initial notmuch new   949.53	697.05	206.98	277436	1290744/21767856
  notmuch new #2        2.12	0.01	0.02	8204	2552/160
  notmuch new #3        0.01	0.00	0.00	8216	0/8
  notmuch new #4        0.01	0.00	0.00	8192	0/8
  notmuch new #5        0.01	0.00	0.00	8216	0/8
  notmuch new #6        0.01	0.00	0.00	8144	0/8

T01-dump-restore.sh: Testing dump and restore           [0.4 large]
			Wall(s)	Usr(s)	Sys(s)	Res(K)	In/Out(512B)
  load nmbug tags       10.78	4.06	3.59	11376	600/39832
  dump *                7.44	6.52	0.91	25296	0/27928
  restore *             7.74	7.24	0.48	8740	0/0

T02-tag.sh: Testing tagging                             [0.4 large]
			Wall(s)	Usr(s)	Sys(s)	Res(K)	In/Out(512B)
  tag * +new_tag        481.78	278.80	196.89	39448	0/1897360
  tag * +existing_tag   0.00	0.00	0.00	8496	0/0
  tag * -existing_tag   449.58	242.65	202.74	35456	0/2073520
  tag * -missing_tag    0.00	0.00	0.00	8440	0/0