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/**
* Copyright (C) 2008 10gen Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the GNU Affero General Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#include "mongo/pch.h"
#include "mongo/db/prefetch.h"
#include "mongo/db/dbhelpers.h"
#include "mongo/db/diskloc.h"
#include "mongo/db/index/index_access_method.h"
#include "mongo/db/catalog/collection.h"
#include "mongo/db/jsobj.h"
#include "mongo/db/repl/rs.h"
#include "mongo/db/stats/timer_stats.h"
#include "mongo/db/commands/server_status_metric.h"
namespace mongo {
// todo / idea: the prefetcher, when it fetches _id, on an upsert, will see if the record exists. if it does not,
// at write time, we can just do an insert, which will be faster.
//The count (of batches) and time spent fetching pages before application
// -- meaning depends on the prefetch behavior: all, _id index, none, etc.)
static TimerStats prefetchIndexStats;
static ServerStatusMetricField<TimerStats> displayPrefetchIndexPages(
"repl.preload.indexes",
&prefetchIndexStats );
static TimerStats prefetchDocStats;
static ServerStatusMetricField<TimerStats> displayPrefetchDocPages(
"repl.preload.docs",
&prefetchDocStats );
// prefetch for an oplog operation
void prefetchPagesForReplicatedOp(Database* db, const BSONObj& op) {
const char *opField;
const char *opType = op.getStringField("op");
switch (*opType) {
case 'i': // insert
case 'd': // delete
opField = "o";
break;
case 'u': // update
opField = "o2";
break;
default:
// prefetch ignores other ops
return;
}
BSONObj obj = op.getObjectField(opField);
const char *ns = op.getStringField("ns");
Collection* collection = db->getCollection( ns );
if ( !collection )
return;
LOG(4) << "index prefetch for op " << *opType << endl;
DEV Lock::assertAtLeastReadLocked(ns);
// should we prefetch index pages on updates? if the update is in-place and doesn't change
// indexed values, it is actually slower - a lot slower if there are a dozen indexes or
// lots of multikeys. possible variations (not all mutually exclusive):
// 1) current behavior: full prefetch
// 2) don't do it for updates
// 3) don't do multikey indexes for updates
// 4) don't prefetchIndexPages on some heuristic; e.g., if it's an $inc.
// 5) if not prefetching index pages (#2), we should do it if we are upsertings and it
// will be an insert. to do that we could do the prefetchRecordPage first and if DNE
// then we do #1.
//
// note that on deletes 'obj' does not have all the keys we would want to prefetch on.
// a way to achieve that would be to prefetch the record first, and then afterwards do
// this part.
//
prefetchIndexPages(collection, obj);
// do not prefetch the data for inserts; it doesn't exist yet
//
// we should consider doing the record prefetch for the delete op case as we hit the record
// when we delete. note if done we only want to touch the first page.
//
// update: do record prefetch.
if ((*opType == 'u') &&
// do not prefetch the data for capped collections because
// they typically do not have an _id index for findById() to use.
!collection->isCapped()) {
prefetchRecordPages(ns, obj);
}
}
void prefetchIndexPages(Collection* collection, const BSONObj& obj) {
DiskLoc unusedDl; // unused
BSONObjSet unusedKeys;
repl::ReplSetImpl::IndexPrefetchConfig prefetchConfig =
repl::theReplSet->getIndexPrefetchConfig();
// do we want prefetchConfig to be (1) as-is, (2) for update ops only, or (3) configured per op type?
// One might want PREFETCH_NONE for updates, but it's more rare that it is a bad idea for inserts.
// #3 (per op), a big issue would be "too many knobs".
switch (prefetchConfig) {
case repl::ReplSetImpl::PREFETCH_NONE:
return;
case repl::ReplSetImpl::PREFETCH_ID_ONLY:
{
TimerHolder timer( &prefetchIndexStats);
// on the update op case, the call to prefetchRecordPages will touch the _id index.
// thus perhaps this option isn't very useful?
try {
IndexDescriptor* desc = collection->getIndexCatalog()->findIdIndex();
if ( !desc )
return;
IndexAccessMethod* iam = collection->getIndexCatalog()->getIndex( desc );
verify( iam );
iam->touch(obj);
}
catch (const DBException& e) {
LOG(2) << "ignoring exception in prefetchIndexPages(): " << e.what() << endl;
}
break;
}
case repl::ReplSetImpl::PREFETCH_ALL:
{
// indexCount includes all indexes, including ones
// in the process of being built
IndexCatalog::IndexIterator ii = collection->getIndexCatalog()->getIndexIterator( true );
while ( ii.more() ) {
TimerHolder timer( &prefetchIndexStats);
// This will page in all index pages for the given object.
try {
IndexDescriptor* desc = ii.next();
IndexAccessMethod* iam = collection->getIndexCatalog()->getIndex( desc );
verify( iam );
iam->touch(obj);
}
catch (const DBException& e) {
LOG(2) << "ignoring exception in prefetchIndexPages(): " << e.what() << endl;
}
unusedKeys.clear();
}
break;
}
default:
fassertFailed(16427);
}
}
void prefetchRecordPages(const char* ns, const BSONObj& obj) {
BSONElement _id;
if( obj.getObjectID(_id) ) {
TimerHolder timer(&prefetchDocStats);
BSONObjBuilder builder;
builder.append(_id);
BSONObj result;
try {
// we can probably use Client::Context here instead of ReadContext as we
// have locked higher up the call stack already
Client::ReadContext ctx( ns );
if( Helpers::findById(ctx.ctx().db(), ns, builder.done(), result) ) {
// do we want to use Record::touch() here? it's pretty similar.
volatile char _dummy_char = '\0';
// Touch the first word on every page in order to fault it into memory
for (int i = 0; i < result.objsize(); i += g_minOSPageSizeBytes) {
_dummy_char += *(result.objdata() + i);
}
// hit the last page, in case we missed it above
_dummy_char += *(result.objdata() + result.objsize() - 1);
}
}
catch(const DBException& e) {
LOG(2) << "ignoring exception in prefetchRecordPages(): " << e.what() << endl;
}
}
}
}
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