journal: introduce entry array chain cache

When traversing entry array chains for a bisection or for retrieving an
item by index we previously always started at the beginning of the
chain. Since we tend to look at the same chains repeatedly, let's cache
where we have been the last time, and maybe we can skip ahead with this
the next time.

This turns most bisections and index lookups from O(log(n)*log(n)) into
O(log(n)). More importantly however, we seek around on disk much less,
which is good to reduce buffer cache and seek times on rotational disks.

1 files changed, 53 insertions, 0 deletions

diff --git a/src/journal/test-journal-enum.c b/src/journal/test-journal-enum.c
new file mode 100644
index 0000000000..8a843ecdda
--- /dev/null
+++ b/src/journal/test-journal-enum.c
@@ -0,0 +1,53 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2012 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <stdio.h>
+
+#include "log.h"
+#include "sd-journal.h"
+
+int main(int argc, char *argv[]) {
+        unsigned n = 0;
+        sd_journal *j;
+
+        log_set_max_level(LOG_DEBUG);
+
+        assert_se(sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY) >= 0);
+
+        assert_se(sd_journal_add_match(j, "_TRANSPORT=syslog", 0) >= 0);
+        assert_se(sd_journal_add_match(j, "_UID=0", 0) >= 0);
+
+        SD_JOURNAL_FOREACH_BACKWARDS(j) {
+                const void *d;
+                size_t l;
+
+                assert_se(sd_journal_get_data(j, "MESSAGE", &d, &l) >= 0);
+
+                printf("%.*s\n", (int) l, (char*) d);
+
+                n ++;
+                if (n >= 10)
+                        break;
+        }
+
+        sd_journal_close(j);
+        return 0;
+}