[PATCH 3/6] xapcmd: reduce CPU idling when shards exceeds job count

[Eric Wong <e@yhbt.net>]

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.
---
 lib/PublicInbox/Xapcmd.pm | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/lib/PublicInbox/Xapcmd.pm b/lib/PublicInbox/Xapcmd.pm
index 6fcc9e90c..b6279218c 100644
--- 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);
 		}
 	}
 }