[RFC PATCHv3 05/10] gitweb/cache.pm - Adaptive cache expiration time

From

Jakub Narebski <jnareb@gmail.com>

Date

Feb 16, 2010, 19:36 UTC

Message-ID

<1266349005-15393-6-git-send-email-jnareb@gmail.com>

In-Reply-To

<1266349005-15393-1-git-send-email-jnareb@gmail.com>

To be implemented (from original patch by J.H.):
* optional locking interface, where only one process can update cache
  (using flock)
* server-side progress indicator when waiting for filling cache,
  which in turn requires separating situations (like snapshots and
  other non-HTML responses) where we should not show 'please wait'
  message

Inspired-by-code-by: John 'Warthog9' Hawley <warthog9@kernel.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
---
Differences from v2:
* Comments about new parameters for the constructor
* More comments
* Access fields directly ($self->{'check_load'}, instead of using accessors
  ($self->get_check_load()).
* Add ->check_load() method (note: does not check if 'check_load' exists).

Differences from relevant parts of J.H. patch:
* 'increase_factor' is configurable rather than hardcoded
* 'check_load' is passed via conctructor parameter; gitweb by default sets
  it to \&get_loadavg.  This means that cache.pm is not entangled with
  gitweb.
* options are stored in %cache_options, e.g. as 'expires_max' (compatible
  with Cache::Adaptive), and not as global variables with un-Perlish
  camelCase names like $maxCacheTime.
* API tests

Possible improvements:
* Turn off adaptive cache lifetime in t9503 test by unsetting
  check_load, and not only by using ->set_expires_in()

 gitweb/cache.pm                 |   80 ++++++++++++++++++++++++++++++++++++--
 gitweb/gitweb.perl              |   27 ++++++++++++-
 t/t9503/test_cache_interface.pl |   22 +++++++++++
 3 files changed, 122 insertions(+), 7 deletions(-)

diff --git a/gitweb/cache.pm b/gitweb/cache.pm
index 151e029..dcddd28 100644
--- a/gitweb/cache.pm
+++ b/gitweb/cache.pm
@@ -57,6 +57,22 @@ our $DEFAULT_CACHE_ROOT = "cache";
 #    'expires_in' (CHI compatibile) [seconds]
 #    The expiration time for objects place in the cache.
 #    Defaults to -1 (never expire) if not explicitly set.
+#    Sets 'expires_min' to given value.
+#  * 'expires_min' [seconds]
+#    The minimum expiration time for objects in cache (e.g. with 0% CPU load).
+#    Used as lower bound in adaptive cache lifetime / expiration.
+#    Defaults to 20 seconds; 'expires_in' sets it also.
+#  * 'expires_max' [seconds]
+#    The maximum expiration time for objects in cache.
+#    Used as upper bound in adaptive cache lifetime / expiration.
+#    Defaults to 1200 seconds, if not set; 
+#    defaults to 'expires_min' if 'expires_in' is used.
+#  * 'check_load'
+#    Subroutine (code) used for adaptive cache lifetime / expiration.
+#    If unset, adaptive caching is turned off; defaults to unset.
+#  * 'increase_factor' [seconds / 100% CPU load]
+#    Factor multiplying 'check_load' result when calculating cache lietime.
+#    Defaults to 60 seconds for 100% SPU load ('check_load' returning 1.0).
 sub new {
 	my ($proto, $p_options_hash_ref) = @_;
 
@@ -64,7 +80,8 @@ sub new {
 	my $self  = {};
 	$self = bless($self, $class);
 
-	my ($root, $depth, $ns, $expires_in);
+	my ($root, $depth, $ns);
+	my ($expires_min, $expires_max, $increase_factor, $check_load);
 	if (defined $p_options_hash_ref) {
 		$root  =
 			$p_options_hash_ref->{'cache_root'} ||
@@ -73,19 +90,31 @@ sub new {
 			$p_options_hash_ref->{'cache_depth'} ||
 			$p_options_hash_ref->{'depth'};
 		$ns    = $p_options_hash_ref->{'namespace'};
-		$expires_in =
+		$expires_min =
+			$p_options_hash_ref->{'expires_min'} ||
 			$p_options_hash_ref->{'default_expires_in'} ||
 			$p_options_hash_ref->{'expires_in'};
+		$expires_max =
+			$p_options_hash_ref->{'expires_max'};
+		$increase_factor = $p_options_hash_ref->{'expires_factor'};
+		$check_load      = $p_options_hash_ref->{'check_load'};
 	}
 	$root  = $DEFAULT_CACHE_ROOT  unless defined($root);
 	$depth = $DEFAULT_CACHE_DEPTH unless defined($depth);
 	$ns    = '' unless defined($ns);
-	$expires_in = -1 unless defined($expires_in); # <0 means never
+	$expires_min =   20 unless defined($expires_min);
+	$expires_max = $expires_min
+		if (!defined($expires_max) && exists $p_options_hash_ref->{'expires_in'});
+	$expires_max = 1200 unless (defined($expires_max));
+	$increase_factor = 60 unless defined($increase_factor);
 
 	$self->set_root($root);
 	$self->set_depth($depth);
 	$self->set_namespace($ns);
-	$self->set_expires_in($expires_in);
+	$self->set_expires_min($expires_min);
+	$self->set_expires_max($expires_max);
+	$self->set_increase_factor($increase_factor);
+	$self->set_check_load($check_load);
 
 	return $self;
 }
@@ -96,7 +125,7 @@ sub new {
 # http://perldesignpatterns.com/perldesignpatterns.html#AccessorPattern
 
 # creates get_depth() and set_depth($depth) etc. methods
-foreach my $i (qw(depth root namespace expires_in)) {
+foreach my $i (qw(depth root namespace expires_min expires_max increase_factor check_load)) {
 	my $field = $i;
 	no strict 'refs';
 	*{"get_$field"} = sub {
@@ -109,6 +138,47 @@ foreach my $i (qw(depth root namespace expires_in)) {
 	};
 }
 
+# ......................................................................
+# pseudo-accessors
+
+# returns adaptive lifetime of cache entry for given $key [seconds]
+sub get_expires_in {
+	my ($self) = @_;
+
+	# short-circuit
+	if (!defined $self->{'check_load'} ||
+	    $self->{'expires_max'} <= $self->{'expires_min'}) {
+		return $self->{'expires_min'};
+	}
+
+	my $expires_in =
+		#$self->{'expires_min'} +
+		$self->{'increase_factor'} * $self->check_load();
+
+	if ($expires_in < $self->{'expires_min'}) {
+		return $self->{'expires_min'};
+	} elsif ($expires_in > $self->{'expires_max'}) {
+		return $self->{'expires_max'};
+	}
+
+	return $expires_in;
+}
+
+# sets expiration time to $duration, turns off adaptive cache lifetime
+sub set_expires_in {
+	my ($self, $duration) = @_;
+
+	$self->{'expires_min'} = $self->{'expires_max'} = $duration;
+}
+
+# runs 'check_load' subroutine, for adaptive cache lifetime.
+# Note: check in caller that 'check_load' exists.
+sub check_load {
+	my $self = shift;
+	#return &{$self->{'check_load'}}();
+	return $self->{'check_load'}->();
+}
+
 # ----------------------------------------------------------------------
 # utility functions and methods
 
diff --git a/gitweb/gitweb.perl b/gitweb/gitweb.perl
index 7073e1b..2b429d2 100755
--- a/gitweb/gitweb.perl
+++ b/gitweb/gitweb.perl
@@ -255,13 +255,36 @@ our %cache_options = (
 	# * File::Spec->catdir(File::Spec->tmpdir(), 'gitweb-cache'),
 	# * '/var/cache/gitweb' (FHS compliant, requires being set up),
 	'cache_root' => 'cache',
+
 	# The number of subdirectories deep to cache object item.  This should be
 	# large enough that no cache directory has more than a few hundred
 	# objects.  Each non-leaf directory contains up to 256 subdirectories
 	# (00-ff).  Must be larger than 0.
 	'cache_depth' => 1,
-	# The (global) expiration time for objects placed in the cache, in seconds.
-	'expires_in' => 20,
+
+	# The (global) minimum expiration time for objects placed in the cache,
+	# in seconds.  If the dynamic adaptive cache exporation time is lower
+	# than this number, we set cache timeout to this minimum.
+	'expires_min' => 20,   # 20 seconds
+
+	# The (global) maximum expiration time for dynamic (adaptive) caching
+	# algorithm, in seconds.  If the adaptive cache lifetime exceeds this
+	# number, we set cache timeout to this maximum.
+	# (If 'expires_min' >= 'expires_max', there is no adaptive cache timeout,
+	# and 'expires_min' is used as expiration time for objects in cache.)
+	'expires_max' => 1200, # 20 minutes
+
+	# Cache lifetime will be increased by applying this factor to the result
+	# from 'check_load' callback (see below).
+	'expires_factor' => 60, # expire time in seconds for 1.0 (100% CPU) load
+
+	# User supplied callback for deciding the cache policy, usually system
+	# load.  Multiplied by 'expires_factor' gives adaptive expiration time,
+	# in seconds, subject to the limits imposed by 'expires_min' and
+	# 'expires_max' bounds.  Set to undef (or delete) to turn off dynamic
+	# lifetime control.
+	# (Compatibile with Cache::Adaptive.)
+	'check_load' => \&get_loadavg,
 );
 
 
diff --git a/t/t9503/test_cache_interface.pl b/t/t9503/test_cache_interface.pl
index 7c7f00c..d28f4df 100755
--- a/t/t9503/test_cache_interface.pl
+++ b/t/t9503/test_cache_interface.pl
@@ -100,6 +100,28 @@ $cache->set($key, $value);
 ok(!defined($cache->get($key)),          'cache is expired');
 $cache->set_expires_in(-1);
 
+# Test assertions for adaptive cache expiration
+#
+my $load = 0.0;
+sub load { return $load; }
+my $expires_min = 10;
+my $expires_max = 30;
+$cache->set_expires_min($expires_min);
+$cache->set_expires_max($expires_max);
+$cache->set_check_load(\&load);
+cmp_ok($cache->get_expires_min(), '==', $expires_min, '"expires min" set correctly');
+cmp_ok($cache->get_expires_max(), '==', $expires_max, '"expires max" set correctly');
+cmp_ok($cache->get_expires_in(), '>=', $expires_min,
+       '"expires in" bound from down for load=0');
+cmp_ok($cache->get_expires_in(), '<=', $expires_max,
+       '"expires in" bound from up   for load=0');
+$load = 1_000;
+cmp_ok($cache->get_expires_in(), '>=', $expires_min,
+       '"expires in" bound from down for heavy load');
+cmp_ok($cache->get_expires_in(), '<=', $expires_max,
+       '"expires in" bound from up   for heavy load');
+$cache->set_expires_in(-1);
+
 # ......................................................................
 
 # Prepare for testing cache_fetch from $cache_pm

-- 
1.6.6.1