From 14e4d410b2606ff5f1e1aa5766a0c2a4f8284072 Mon Sep 17 00:00:00 2001
From: Michal Schmidt <mschmidt@redhat.com>
Date: Tue, 9 Dec 2014 20:27:19 +0100
Subject: [PATCH] journal: optimize iteration: skip whole files behind current
 location

Interleaving of entries from many journal files is expensive. But there
is room for optimization.

We can skip looking into journal files whose entries all lie before the
current iterating location. We can tell if that's the case from looking
at the journal file header. This saves a huge amount of work if one has
many of mostly not interleaved journal files.

On my workstation with 90 journal files in /var/log/journal/ID/
totalling 3.4 GB I get these results:

Before:
$ time ./journalctl --since=2014-06-01 --until=2014-07-01 > /dev/null

real    5m54.258s
user    2m4.263s
sys     3m48.965s

After:
$ time ./journalctl --since=2014-06-01 --until=2014-07-01 > /dev/null

real    0m20.518s
user    0m19.989s
sys     0m0.328s

The high "sys" time in the original was caused by putting more stress on
the mmap-cache than it could handle. With the patch the working set
now consists of fewer mmap windows and mmap-cache is not thrashing.

(cherry picked from commit b7c88ab8cc7d55a43450bf3dea750f95f2e910d6)
---
 src/journal/sd-journal.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/src/journal/sd-journal.c b/src/journal/sd-journal.c
index cf21c4d899..2df147c02a 100644
--- a/src/journal/sd-journal.c
+++ b/src/journal/sd-journal.c
@@ -497,6 +497,26 @@ static int compare_entry_order(JournalFile *af, Object *_ao,
         return 0;
 }
 
+static bool whole_file_precedes_location(JournalFile *f, Location *l, direction_t direction) {
+        assert(f);
+        assert(l);
+
+        if (l->type != LOCATION_DISCRETE && l->type != LOCATION_SEEK)
+                return false;
+
+        if (l->seqnum_set && sd_id128_equal(l->seqnum_id, f->header->seqnum_id))
+                return direction == DIRECTION_DOWN ?
+                        l->seqnum > le64toh(f->header->tail_entry_seqnum) :
+                        l->seqnum < le64toh(f->header->head_entry_seqnum);
+
+        if (l->realtime_set)
+                return direction == DIRECTION_DOWN ?
+                        l->realtime > le64toh(f->header->tail_entry_realtime) :
+                        l->realtime < le64toh(f->header->head_entry_realtime);
+
+        return false;
+}
+
 _pure_ static int compare_with_location(JournalFile *af, Object *ao, Location *l) {
         uint64_t a;
 
@@ -882,6 +902,9 @@ static int real_journal_next(sd_journal *j, direction_t direction) {
         ORDERED_HASHMAP_FOREACH(f, j->files, i) {
                 bool found;
 
+                if (whole_file_precedes_location(f, &j->current_location, direction))
+                        continue;
+
                 r = next_beyond_location(j, f, direction, &o, &p);
                 if (r < 0) {
                         log_debug("Can't iterate through %s, ignoring: %s", f->path, strerror(-r));