index: v2: --sequential-shard option

This gives better page cache utilization for Xapian indexing on
slow storage by improving locality for random I/O activity on
the Xapian DB.

Instead of doing a single-pass to index both SQLite and Xapian;
this indexes them separately.  The first pass is identical to
indexlevel=basic: it indexes both over.sqlite3 and msgmap.sqlite3.

Subsequent passes only operate on a single Xapian shard for
documents belonging to that shard.  Given enough shards, each
individual shard can be made small enough to fit into the kernel
page cache and avoid HDD seeks for read activity.

Doing rough tests with a busy system with a 7200 RPM HDD with ext4,
full indexing of LKML (9 epochs) goes from ~80 hours (-j0) to
~30 hours (-j8) with 16GB RAM with 7 shards configured and fsync(2)
disabled (--no-sync) and `--batch-size=10m'.

1 files changed, 52 insertions, 3 deletions

diff --git a/lib/PublicInbox/V2Writable.pm b/lib/PublicInbox/V2Writable.pm
index f98afa61..7bc24592 100644
--- a/lib/PublicInbox/V2Writable.pm
+++ b/lib/PublicInbox/V2Writable.pm
@@ -875,7 +875,8 @@ sub reindex_checkpoint ($$) {
 
         $self->{ibx}->git->cleanup; # *async_wait
         ${$sync->{need_checkpoint}} = 0;
-        $sync->{mm_tmp}->atfork_prepare;
+        my $mm_tmp = $sync->{mm_tmp};
+        $mm_tmp->atfork_prepare if $mm_tmp;
         $self->done; # release lock
 
         if (my $pr = $sync->{-opt}->{-progress}) {
@@ -884,7 +885,7 @@ sub reindex_checkpoint ($$) {
 
         # allow -watch or -mda to write...
         $self->idx_init; # reacquire lock
-        $sync->{mm_tmp}->atfork_parent;
+        $mm_tmp->atfork_parent if $mm_tmp;
 }
 
 sub index_oid { # cat_async callback
@@ -1085,7 +1086,10 @@ sub sync_prepare ($$$) {
                 }
                 $all->cat_async_wait;
         }
-        return 0 if (!$regen_max && !keys(%{$self->{unindex_range}}));
+        if (!$regen_max && !keys(%{$self->{unindex_range}})) {
+                $sync->{-regen_fmt} = "%u/?\n";
+                return 0;
+        }
 
         # reindex should NOT see new commits anymore, if we do,
         # it's a problem and we need to notice it via die()
@@ -1177,6 +1181,36 @@ sub sync_ranges ($$$) {
         $ranges;
 }
 
+sub index_xap_only { # git->cat_async callback
+        my ($bref, $oid, $type, $size, $smsg) = @_;
+        my $self = $smsg->{v2w};
+        my $idx = idx_shard($self, $smsg->{num} % $self->{shards});
+        $idx->begin_txn_lazy;
+        $idx->add_message(PublicInbox::Eml->new($bref), $smsg);
+        $self->{transact_bytes} += $size;
+}
+
+sub index_seq_shard ($$$) {
+        my ($self, $sync, $off) = @_;
+        my $ibx = $self->{ibx};
+        my $max = $ibx->mm->max or return;
+        my $all = $ibx->git;
+        my $over = $ibx->over;
+        my $batch_bytes = $PublicInbox::SearchIdx::BATCH_BYTES;
+        if (my $pr = $sync->{-opt}->{-progress}) {
+                $pr->("Xapian indexlevel=$ibx->{indexlevel} % $off\n");
+        }
+        for (my $num = $off; $num <= $max; $num += $self->{shards}) {
+                my $smsg = $over->get_art($num) or next;
+                $smsg->{v2w} = $self;
+                $all->cat_async($smsg->{blob}, \&index_xap_only, $smsg);
+                if ($self->{transact_bytes} >= $batch_bytes) {
+                        ${$sync->{nr}} = $num;
+                        reindex_checkpoint($self, $sync);
+                }
+        }
+}
+
 sub index_epoch ($$$) {
         my ($self, $sync, $i) = @_;
 
@@ -1218,6 +1252,11 @@ sub index_sync {
         my $epoch_max;
         my $latest = git_dir_latest($self, \$epoch_max);
         return unless defined $latest;
+
+        my $seq = $opt->{sequentialshard};
+        my $idxlevel = $self->{ibx}->{indexlevel};
+        local $self->{ibx}->{indexlevel} = 'basic' if $seq;
+
         $self->idx_init($opt); # acquire lock
         fill_alternates($self, $epoch_max);
         $self->{over}->rethread_prepare($opt);
@@ -1252,6 +1291,16 @@ sub index_sync {
                 $pr->('all.git '.sprintf($sync->{-regen_fmt}, $$nr)) if $pr;
         }
 
+        if ($seq) { # deal with Xapian shards sequentially
+                my $end = $self->{shards} - 1;
+                $self->{ibx}->{indexlevel} = $idxlevel;
+                delete $sync->{mm_tmp};
+                $self->idx_init($opt); # re-acquire lock
+                index_seq_shard($self, $sync, $_) for (0..$end);
+                $self->{ibx}->git->cat_async_wait;
+                $self->done;
+        }
+
         # reindex does not pick up new changes, so we rerun w/o it:
         if ($opt->{reindex}) {
                 my %again = %$opt;