path: root/src/mongo/db/prefetch.cpp

/**
*    Copyright (C) 2008-2014 MongoDB Inc.
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*    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)
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#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kReplication

#include "mongo/platform/basic.h"

#include "mongo/db/prefetch.h"

#include "mongo/db/catalog/collection.h"
#include "mongo/db/catalog/database.h"
#include "mongo/db/commands/server_status_metric.h"
#include "mongo/db/dbhelpers.h"
#include "mongo/db/index/index_access_method.h"
#include "mongo/db/jsobj.h"
#include "mongo/db/repl/bgsync.h"
#include "mongo/db/repl/replication_coordinator.h"
#include "mongo/db/repl/replication_coordinator_global.h"
#include "mongo/db/server_parameters.h"
#include "mongo/db/stats/timer_stats.h"
#include "mongo/db/storage/mmap_v1/mmap.h"
#include "mongo/util/log.h"

namespace mongo {

using std::endl;
using std::string;

namespace repl {
namespace {
// 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.)
TimerStats prefetchIndexStats;
ServerStatusMetricField<TimerStats> displayPrefetchIndexPages("repl.preload.indexes",
                                                              &prefetchIndexStats);
TimerStats prefetchDocStats;
ServerStatusMetricField<TimerStats> displayPrefetchDocPages("repl.preload.docs", &prefetchDocStats);

// page in pages needed for all index lookups on a given object
void prefetchIndexPages(OperationContext* txn,
                        Collection* collection,
                        const BackgroundSync::IndexPrefetchConfig& prefetchConfig,
                        const BSONObj& obj) {
    // 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 BackgroundSync::PREFETCH_NONE:
            return;
        case BackgroundSync::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(txn);
                if (!desc)
                    return;
                IndexAccessMethod* iam = collection->getIndexCatalog()->getIndex(desc);
                invariant(iam);
                iam->touch(txn, obj);
            } catch (const DBException& e) {
                LOG(2) << "ignoring exception in prefetchIndexPages(): " << e.what() << endl;
            }
            break;
        }
        case BackgroundSync::PREFETCH_ALL: {
            // indexCount includes all indexes, including ones
            // in the process of being built
            IndexCatalog::IndexIterator ii =
                collection->getIndexCatalog()->getIndexIterator(txn, 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(txn, obj);
                } catch (const DBException& e) {
                    LOG(2) << "ignoring exception in prefetchIndexPages(): " << e.what() << endl;
                }
            }
            break;
        }
        default:
            fassertFailed(16427);
    }
}

// page in the data pages for a record associated with an object
void prefetchRecordPages(OperationContext* txn, Database* db, const char* ns, const BSONObj& obj) {
    BSONElement _id;
    if (obj.getObjectID(_id)) {
        TimerHolder timer(&prefetchDocStats);
        BSONObjBuilder builder;
        builder.append(_id);
        BSONObj result;
        try {
            if (Helpers::findById(txn, 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;
        }
    }
}
}  // namespace

// prefetch for an oplog operation
void prefetchPagesForReplicatedOp(OperationContext* txn, Database* db, const BSONObj& op) {
    invariant(db);
    const BackgroundSync::IndexPrefetchConfig prefetchConfig =
        BackgroundSync::get()->getIndexPrefetchConfig();
    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");

    // This will have to change for engines other than MMAP V1, because they might not have
    // means for directly prefetching pages from the collection. For this purpose, acquire S
    // lock on the database, instead of optimizing with IS.
    Lock::CollectionLock collLock(txn->lockState(), ns, MODE_S);

    Collection* collection = db->getCollection(ns);
    if (!collection) {
        return;
    }

    LOG(4) << "index prefetch for op " << *opType << endl;

    // 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(txn, collection, prefetchConfig, 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(txn, db, ns, obj);
    }
}

class ReplIndexPrefetch : public ServerParameter {
public:
    ReplIndexPrefetch() : ServerParameter(ServerParameterSet::getGlobal(), "replIndexPrefetch") {}

    virtual ~ReplIndexPrefetch() {}

    const char* _value() {
        if (getGlobalReplicationCoordinator()->getReplicationMode() !=
            ReplicationCoordinator::modeReplSet) {
            return "uninitialized";
        }
        BackgroundSync::IndexPrefetchConfig ip = BackgroundSync::get()->getIndexPrefetchConfig();
        switch (ip) {
            case BackgroundSync::PREFETCH_NONE:
                return "none";
            case BackgroundSync::PREFETCH_ID_ONLY:
                return "_id_only";
            case BackgroundSync::PREFETCH_ALL:
                return "all";
            default:
                return "invalid";
        }
    }

    virtual void append(OperationContext* txn, BSONObjBuilder& b, const string& name) {
        b.append(name, _value());
    }

    virtual Status set(const BSONElement& newValueElement) {
        if (getGlobalReplicationCoordinator()->getReplicationMode() !=
            ReplicationCoordinator::modeReplSet) {
            return Status(ErrorCodes::BadValue, "replication is not enabled");
        }

        std::string prefetch = newValueElement.valuestrsafe();
        return setFromString(prefetch);
    }

    virtual Status setFromString(const string& prefetch) {
        log() << "changing replication index prefetch behavior to " << prefetch << endl;

        BackgroundSync::IndexPrefetchConfig prefetchConfig;

        if (prefetch == "none")
            prefetchConfig = BackgroundSync::PREFETCH_NONE;
        else if (prefetch == "_id_only")
            prefetchConfig = BackgroundSync::PREFETCH_ID_ONLY;
        else if (prefetch == "all")
            prefetchConfig = BackgroundSync::PREFETCH_ALL;
        else {
            return Status(ErrorCodes::BadValue,
                          str::stream() << "unrecognized indexPrefetch setting: " << prefetch);
        }

        BackgroundSync::get()->setIndexPrefetchConfig(prefetchConfig);
        return Status::OK();
    }

} replIndexPrefetch;

}  // namespace repl
}  // namespace mongo