xapcmd: reduce CPU idling when shards exceeds job count

In case there's unbalanced shards AND we're limiting parallelism
while using many shards, spawn the next task in the queue ASAP
once a task is done, instead of waiting for all tasks to finish
before spawning the next batch.

Unbalanced shards probably isn't a big issue for most users;
however many smaller shards with few jobs can be useful for HDD
users to reduce the effect of random writes.

diff --git a/lib/PublicInbox/Xapcmd.pm b/lib/PublicInbox/Xapcmd.pm
index 6fcc9e90cd42e45d22eee17993bf2edc84fec028..b6279218c880880fbcc3b3930dad29b2e3fc8471 100644 (file)
--- a/lib/PublicInbox/Xapcmd.pm
+++ b/lib/PublicInbox/Xapcmd.pm
@@ -9,7 +9,7 @@ use PublicInbox::SearchIdx;
 use File::Temp 0.19 (); # ->newdir
 use File::Path qw(remove_tree);
 use File::Basename qw(dirname);
-use POSIX ();
+use POSIX qw(WNOHANG);
 
 # support testing with dev versions of Xapian which installs
 # commands with a version number suffix (e.g. "xapian-compact-1.5")
@@ -151,14 +151,17 @@ sub process_queue {
                        $pids{cb_spawn($cb, $args, $opt)} = $args;
                }
 
+               my $flags = 0;
                while (scalar keys %pids) {
-                       my $pid = waitpid(-1, 0);
+                       my $pid = waitpid(-1, $flags) or last;
+                       last if $pid < 0;
                        my $args = delete $pids{$pid};
                        if ($args) {
                                die join(' ', @$args)." failed: $?\n" if $?;
                        } else {
                                warn "unknown PID($pid) reaped: $?\n";
                        }
+                       $flags = WNOHANG if scalar(@$queue);
                }
        }
 }