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Re: Slapd frontend performance issues

Hello Sam!

You have clearly pointed out all well-known deficiences - unfortunately I'm
not so good in English :) These things were discussed, but not fixed due to
limited resources.

If you can provide a patch against current code solving all these issues -
that would be *great*.
If you decide to contribute, please post the patch through ITS, along with
your copyright statement, and I will be happy to commit it to development
branch. If you also could modify/clearify docs and samples accordingly, it
would be event greater :)
Thank you in advance.

As for future optimizations - as I alreday said, I see the primary step as a
bit of profiling, both the frontend (with dummy backend), and with various
backends, to make it clear where to start. There are so many potential
improvements, and some of them require plenty of coding, while it may appear
that they don't contribute much to the performance.

WBW, Dmitry

----- Original Message -----
From: "Sam Drake" <drake@timesten.com>
To: <mitya@seismic.ru>
Cc: <openldap-devel@OpenLDAP.org>
Sent: Sunday, April 08, 2001 9:39 AM
Subject: RE: Slapd frontend performance issues

> FYI, here is a short description of the changes I made.  I'll package up
> changes asap, but it may take a couple of days.
> The performance numbers quoted in this report were seen at my location
> a 100,000 object database ... the slower numbers I mentioned earlier were
> reported by a customer with a 1,000,000 object database.
> I also can't explain the very poor performance I saw with OpenLDAP and
> with a 100,000 object database.
> ...Sam Drake / TimesTen Performance Software
> ----------
> Work Performed
> OpenLDAP 2.0.9, including back-sql, was built successfully on Solaris
> 8 using gcc.  The LDAP server itself, slapd, passed all tests bundled
> with OpenLDAP.  OpenLDAP was built using Sleepycat LDBM release 3.1.17
> as the "native" storage manager.
> The experimental back-sql facility in slapd was also built
> successfully.  It was built using Oracle release 8.1.7 and the Oracle
> ODBC driver and ODBC Driver Manager from Merant.  Rudimentary testing
> was performed with the data and examples provided with back-sql, and
> back-sql was found to be functional.
> Slapd and back-sql were then tested with TimesTen, using TimesTen
> 4.1.1.  Back-sql was not immediately functional with TimesTen due to a
> number of SQL limitations in the TimesTen product.
> Functional issues encountered were:
> 1. Back-sql issued SELECT statements including the construct,
>    "UPPER(?)".  While TimesTen supports UPPER, it does not support the
>    use of parameters as input to builtin functions.  Back-sql was
>    modified to convert the parameter to upper case prior to giving it
>    to the underlying database ... a change that is appropriate for all
>    databases.
> 2. Back-sql issued SELECT statements using the SQL CONCAT function.
>    TimesTen does not support this function.  Back-sql was modified to
>    concatentate the necessary strings itself (in "C" code) prior to
>    passing the parameters to SQL.  This change is also appropriate for
>    all databases, not just TimesTen.
> Once these two issues were resolved, back-sql could successfully
> process LDAP searches using the sample data and examples provided with
> back-sql.
> While performance was not measured at this point, numerous serious
> performance problems were observed with the back-sql code and the
> generated SQL.  In particular:
> 1. In the process of implementing an LDAP search, back-sql will
>    generate and execute a SQL query for all object classes stored in
>    back-sql.  During the source of generating each SQL query, it is
>    common for back-sql to determine that a particular object class can
>    not possibly have any members satisfying the search.  For example,
>    this can occur if the query searches an attribute of the LDAP
>    object that does not exist in the SQL schema.  In this case,
>    back-sql would generate and issue the SQL query anyway, including a
>    clause such as "WHERE 1=0" in the generated SELECT.  The overhead
>    of parsing, optimizing and executing the query is non-trivial, and
>    the answer (the empty set) is known in advance. Solution: Back-sql
>    was modified to stop executing a SQL query when it can be
>    predetermined that the query will return no rows.
> 2. Searches in LDAP are fundamentally case-insensitive ("abc" is equal
>    to "aBc").  However, in SQL this is not normally the case.
>    Back-sql thus generated SQL SELECT statements including clauses of
>    the form, "WHERE UPPER(attribute) = 'JOE'".  Even if an index is
>    defined on the attribute in the relational database, the index can
>    not be used to satisfy the query, as the index is case sensitive.
>    The relational database then is forced to scan all rows in the
>    table in order to satisfy the query ... an expensive and
>    non-scalable proposition.  Solution: Back-sql was modified to allow
>    the schema designer to add additional "upper cased" columns to the
>    SQL schema.  These columns, if present, contain an upper cased
>    version of the "standard" field, and will be used preferentially
>    for searching.  Such columns can be provided for all searchable
>    columns, some columns, or no columns.  An application using
>    database "triggers" or similar mechanisms can automatically
>    maintain these upper cased columns when the standard column is
>    changed.
> 3. In order to implement the hierarchical nature of LDAP object
>    hierarchies, OpenLDAP uses suffix searches in SQL.  For example, to
>    find all objects in the subtree "o=TimesTen,c=us", a SQL SELECT
>    statement of the form, "WHERE UPPER(dn) LIKE '%O=TIMESTEN,C=US'"
>    would be employed.  Aside from the UPPER issue discussed above, a
>    second performance problem in this query is the use of suffix
>    search.  In TimesTen (and most relational databases), indexes can
>    be used to optimize exact-match searches and prefix searches.
>    However, suffix searches must be performed by scanning every row in
>    the table ... an expensive and non-scalable proposition.  Solution:
>    Back-sql was modified to optionally add a new "dn_ru" column to the
>    ldap_entries table.  This additional column, if present, contains a
>    byte-reversed and upper cased version of the DN.  This allows
>    back-sql to generate indexable prefix searches.  This column is
>    also easily maintained automatically through the use of triggers.
> Results
> A simple database schema was generated holding the LDAP objects and
> attributes specified by our customer.  An application was written to
> generate test databases.  Both TimesTen and Oracle 8.1.7 were
> populated with 100,000 entry databases.
> Load Times
> Using "slapadd" followed by "slapindex", loading and indexing 100,000
> entries in an LDBM database ran for 19 minutes 10 seconds.
> Using a C++ application that used ODBC, loading 100,000 entries into
> a disk based RDBMS took 17 minutes 53 seconds.
> Using a C++ application that used ODBC, loading 100,000 entries into
> TimesTen took 1 minute 40 seconds.
> Search Times
> The command, "timex timesearch.sh '(cn=fname210100*)'" was used to
> test search times.  This command issues the same LDAP search 4000
> times over a single LDAP connection.  Both the client and server
> (slapd) were run on the same machine.
> With TimesTen as the database, 4000 queries took 14.93 seconds, for a
> rate of 267.9 per second.
> With a disk based RDBMS as the database, 4000 queries took 77.79 seconds,
> for a
> rate of 51.42 per second.
> With LDBM as the database, 1 query takes 76 seconds, or 0.076 per
> second.  Something is clearly broken.