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Item13525: Store reads the same file repeatedly from disk

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Priority: Normal
Current State: Waiting for Feedback
Released In: n/a
Target Release: patch
Applies To: Extension
Component: RCSStoreContrib
Branches: Item13525
Reported By: MichaelDaum
Waiting For: MichaelDaum
Last Change By: MichaelDaum
Investigating the most expensive calls in Foswiki-2.0.0 using nytprof shows that Foswiki::Store::Rcs::Handler::readFile() is called up to 18 times for the same file (PatterSkinNavigation) and overall 118 files when visiting System.WebHome. That's quite a lot. Disk io is THE major bottleneck unless you are using SSDs.

This is a low-hanging fruit to lower disk-io using below simple patch:

diff --git a/RCSStoreContrib/lib/Foswiki/Store/Rcs/Handler.pm b/RCSStoreContrib/lib/Foswiki/Store/Rcs/Handler.pm
index 4340f77..9c1bc1b 100644
--- a/RCSStoreContrib/lib/Foswiki/Store/Rcs/Handler.pm
+++ b/RCSStoreContrib/lib/Foswiki/Store/Rcs/Handler.pm
@@ -152,6 +152,7 @@ sub finish {
     undef $this->{web};
     undef $this->{topic};
     undef $this->{attachment};
+    undef $this->{cache};
 }
 
 # Used in subclasses for late initialisation during object creation
@@ -1113,6 +1114,9 @@ sub saveFile {
     close($fh)
       or throw Error::Simple(
         'Rcs::Handler: failed to close file ' . $name . ': ' . $! );
+
+    $this->{cache}{$name} = undef;
+
     return;
 }
 
@@ -1120,7 +1124,10 @@ sub saveFile {
 sub readFile {
     my ( $this, $name ) = @_;
     ASSERT($name) if DEBUG;
-    my $data;
+
+    my $data = $this->{cache}{$name};
+    return $data if defined $data;
+
     my $IN_FILE;
 
     # Note: no IO layer; we want to trap encoding errors
@@ -1130,6 +1137,9 @@ sub readFile {
         $data = <$IN_FILE>;
         close($IN_FILE);
     }
+
+    $this->{cache}{$name} = $data;
+
     return $data;
 }

As a side, note I found out that Foswiki::Store::Rcs::Handler::getRevision($version) ignores the $version parameter and always returns the top revision as stored in the .txt file.

-- MichaelDaum - 15 Jul 2015

Patch updated to invalidate it when saveFile is called.

-- MichaelDaum - 17 Jul 2015

Changed attribution to RCSStoreContrib. I'm not convinced this is Urgent - no details of what operation on what topic with what skin in order to benchmark frown, sad smile

-- Main.CrawfordCurrie - 20 Jul 2015 - 06:52

Skin does not matter, topic is System.WebHome as mentioned above, operation is view.

-- MichaelDaum - 20 Jul 2015

After digging a bit into Kenneth's report Item13593, it seems that the MetaCache doesn't cache very much. I've got a patch pasted into that topic that seems to improve that cache by refusing to re-add the same topic into the cache.

I suspect that these two problems are related. By not taking advantage of the MetaCache, that will certainly cause extra topic reads. In addition it means parsing the topic meta again which is also quite expensive. Fixing the MetaCache is probably lower risk than adding another cache in the store.

-- GeorgeClark - 31 Jul 2015

The fix in Item13593 had very significant improvement. I think that this item could be closed as a duplicate. Please see if the MetaCache was effective in eliminating the extra I/O that this patch resolved.

-- GeorgeClark - 05 Aug 2015

I will have to re-profile again. Above patch may now indeed only have negligible value. Having less pressure on the disk - no matter how - is a good thing.

-- MichaelDaum - 06 Aug 2015

Downgrading prio until the issue has been re-analyzed again.

-- MichaelDaum - 10 Aug 2015
 

ItemTemplate edit

Summary Store reads the same file repeatedly from disk
ReportedBy MichaelDaum
Codebase 2.0.1, 2.0.0
SVN Range
AppliesTo Extension
Component RCSStoreContrib
Priority Normal
CurrentState Waiting for Feedback
WaitingFor MichaelDaum
Checkins distro:5735e2d33652
TargetRelease patch
ReleasedIn n/a
CheckinsOnBranches Item13525
trunkCheckins
masterCheckins
ItemBranchCheckins distro:5735e2d33652
Release01x01Checkins
Topic attachments
I Attachment Action Size Date Who Comment
profile_foswikiEXT profile_foswiki manage 445 bytes 20 Jul 2015 - 13:17 MichaelDaum script to run foswiki using the nyt profiler
Topic revision: r10 - 10 Aug 2015, MichaelDaum - This page was cached on 25 Feb 2018 - 19:36.

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