Monday Mar 02, 2009

New DTrace Probes in PostgreSQL 8.4

DTrace probes were introduced in PostgreSQL starting in 8.2. Initially, only a handful were added, and they were mostly for developers. In 8.4, many more (46 to be exact) have been added, and they are targeted more toward database administrators as shown below.

query-parse-start(const char \*)
query-parse-done(const char \*)
query-rewrite-start(const char \*)
query-rewrite-done(const char \*)
query-start(const char \*)
query-done(const char \*)
statement-status(const char \*)
sort-start(int, bool, int, int, bool)
sort-done(unsigned long, long)
buffer-read-start(ForkNumber, BlockNumber, Oid, Oid, Oid, bool)
buffer-read-done(ForkNumber, BlockNumber, Oid, Oid, Oid, bool, bool)buffer-flush-start(Oid, Oid, Oid)
buffer-flush-done(Oid, Oid, Oid)
buffer-sync-start(int, int)
buffer-sync-done(int, int, int)
buffer-write-dirty-start(ForkNumber, BlockNumber, Oid, Oid, Oid)
buffer-write-dirty-done(ForkNumber, BlockNumber, Oid, Oid, Oid)
checkpoint-done(int, int, int, int, int)
smgr-md-read-start(ForkNumber, BlockNumber, Oid, Oid, Oid)
smgr-md-read-done(ForkNumber, BlockNumber, Oid, Oid, Oid, const char \*, int, int)
smgr-md-write-start(ForkNumber, BlockNumber, Oid, Oid, Oid)
smgr-md-write-done(ForkNumber, BlockNumber, Oid, Oid, Oid, const char \*, int, int)
xlog-insert(unsigned char, unsigned char)

Documentation should be available in 8.4 doc page soon, but if you don't want to wait, check out the doc patch I recently submitted. If you're using or plan to use the probes, I'd love to hear your feedback, both positive and constructive!

Special thanks to Theo Schlossnagle, Robert Treat, Zdenek Kotala, Alvaro Herrera and Simon Riggs for their contributions with the probes as well as reviewing them.

Thursday Apr 03, 2008

DTrace probes in PostgreSQL now work on Mac OS X Leopard

The issue with PostgreSQL's DTrace probes not working with Mac OS X Leopard as reported in the mail list has been fixed and checked into the 8.4 development tree. The problem had to do with the fact that Leopard's DTrace implementation not supporting the -G flag.

If you're curious about the gory details, check out the proposal, patch, and code commit.

With the new implementation, the steps for adding new probes are slightly different than before, but the provider and probe names remain the same. For details on how to use and add new probes, refer to the online doc.

Many thanks to Peter Eisentraut, Tom Lane, and Alvaro Herrera for their valuable feedback and assistance!

Tuesday Jun 12, 2007

PostgreSQL 8.2.4 in Solaris Expres Developer Edition

Today Sun announced the availability of Solaris Express Developer Edition 5/07. There are many new features in this release and among them is the inclusion of PostgreSQL 8.2.4 with SMF and DTrace integration.

Here's how you'd run Postgres 8.2:
    1) As root, su to postgres
    # su - postgres

    2) Create Postgres DB cluster
    $ /usr/postgres/8.2/bin/initdb -D /var/postgres/8.2/data

    3) As root, use the SMF's svadm command to start Postgres
    # /usr/sbin/svcadm enable postgresql:version_82

Note that Postgres 8.1 is also available. The binaries are located in /usr/bin and /usr/postgres/8.2/bin for 8.1 & 8.2 respectively. To use 8.2, make sure to add /usr/postgres/8.2/bin in the PATH. For more info see postgres_82 man page (e.g. run "man postgres_82" from the command prompt).

With 8.2.4 all the user-level DTrace probes are now enabled. To see the list of available probes, run "dtrace -l | grep postgres"

For more info on how to use the probes, refer to

Sunday Aug 27, 2006

User-Level DTrace Probes in PostgreSQL

I'm excited to announce that PostgreSQL 8.2 now has user-level DTrace probes embedded in the source code. These probes will enable users to easily observe the behavior of PostgreSQL with simple D scripts, even in production. As you may already know, DTrace is being ported to FreeBSD and Mac OS X, so PostgreSQL users will be able to use the embedded probes on these OSes as well besides Solaris. My hope is that the presence of DTrace probes will not only help users identify one-off performance problem but will also enable developers to identify systemic performance and scalability issues on big multi-cpu/core/thread systems.

Here's the current list of available probes.
provider postgresql {
        probe transaction__start(int);
        probe transaction__commit(int);
        probe transaction__abort(int);
        probe lwlock__acquire(int, int);
        probe lwlock__release(int); 
        probe lwlock__startwait(int, int);
        probe lwlock__endwait(int, int);
        probe lwlock__condacquire(int, int);
        probe lwlock__condacquire__fail(int, int);
        probe lock__startwait(int, int);
        probe lock__endwait(int, int);

As you can see, the number of probes is small initially, but more will be added over time, and I encourage the community to identify areas in PostgreSQL where more observability is needed, for both developers and admins.

PostgreSQL runs on many operating systems, and the community is quite strict about keeping the code generic. To accomodate this, we created a higher level of abstraction whereby generic macro names are used instead of the DTrace specific macros. For example, we define the following macros PG_TRACE, PG_TRACE1, etc. which ultimately translate to DTRACE_PROBE, DTRACE_PROBE1, ... when used on system with DTrace. Doing this allow the tracing code to use generic macro names and these macros can be mapped to other tracing facilities for other operating systems.

The next few sections explains how to:
  • Compile PostgreSQL with DTrace
  • Use the existing DTrace probes
  • Add new DTrace probes

Compile PostgreSQL with DTrace

By default DTrace probes are disabled, and the user needs to explicitly tell the configure script to make the probes available in PostgreSQL. Certainly, enabling DTrace only makes sense on Operating Systems with DTrace facility. Currently DTrace is available on Solaris 10+ and soon on FreeBSD and Mac OS X.

To include DTrace probes in a 32 bit binary, specify --enable-dtrace to configure. For example:
        $ configure --enable-dtrace ...

To include DTrace probes in a 64 bit binary, specify --enable-dtrace and DTRACEFLAGS="-64" to configure. For example:

         Using gcc compiler:
        $ configure CC='gcc -m64' --enable-dtrace DTRACEFLAGS='-64' ...
         Using Sun compiler:
        $ configure CC='/path_to_sun_compiler/cc -xtarget=native64' --enable-dtrace DTRACEFLAGS='-64' ...

a) To successfully compile PostgreSQL 8.2 with --enable-dtrace, you need to run Solaris Express. The DTrace version in Solaris 10 (up until 11/06) does not allow probes to be added to static functions. This limitation will be fixed in the next update of Solaris 10.
b) When using DTRACEFLAGS='-64', you also have to tell the compiler to build 64 bit binary as shown in the configure lines above; otherwise, you will get compilation errors.

Use Existing DTrace Probes

Using the probes in PostgreSQL is similar to using probes in other DTrace providers. Below is an example of a simple D script using the transaction-start, transaction-commit, and transaction-abort probes. The script prints out the total number of started, committed, and aborted transactions.

#!/usr/sbin/dtrace -qs 

        @start["Start"] = count();
        self->ts  = timestamp;

        @abort["Abort"] = count();

        @commit["Commit"] = count();
        @time["Total time (ns)"] = sum(timestamp - self->ts);

Executing the above script produces the following output.

# ./txn_count.d `pgrep -n postgres`

  Start                                         71
  Commit                                   70
  Total time (ns)                        2312105013

A number of sample D scripts are available from the DTrace's PgFoundry project

To learn more about DTrace, refer to the HowTo and DTrace Guides.

Add New DTrace Probes

New DTrace probes can easily be added to PostgreSQL. For example, if you were to add transaction-start probe, follow these simple steps:

1) Add the probe definitions to src/backend/utils/probes.d
   provider postgresql {
        probe transaction__start(int);

When a dash (-) is used in the probe name, it needs to be converted to double underscores (__) in the probe definition file. So, the above probe is called transaction-start in the D script.

2) Add "PG_TRACE1 (transaction__start, s->transactionId);" to backend/access/transam/xact.c
   static void

         \* generate a new transaction id
        s->transactionId = GetNewTransactionId(false);


        PG_TRACE1 (transaction__start, s->transactionId);


a) PG_TRACE1 is mapped to DTRACE_PROBE1. See src/include/pg_trace.h
b) The provider name for all probes in PostgreSQL is called postgresql per the decision by the developers, so it's not specified in PG_TRACE. See src/include/pg_trace.h.
c) Make sure the data types in the probe definition match the argument passed to the probe. In this case s->transactionId has to be an integer (int).

When you have probes that might be useful to the community at large, send a proposal/patch to to get feedback from the developers.

3) Check to make sure the new probe is available

After recompiling, run the new binary, and as root, execute the following DTrace command to check that your newly added probe is available.

# dtrace -l -n transaction-start

More details which led to DTrace inclusion in PostgreSQL

1) Proposal submitted to the developer community.
2) Presented the proposal at the PostgreSQL Anniversary Summit (first developers conference). The timing of the conference was perfect, and I was fortunate to have the opportunity to present the proposal and demo'ed DTrace to a live audience. I think the discussion with the developers/hackers after the conference was key in solidifying the proposed implementation and getting the thumbs up for inclusion into 8.2.
3) Patch submitted and follow-on discussions
4) Patch was updated by Peter Eisentraut
5) Patch was finally committed by Peter Eisentraut


Many people from the community and at Sun provided excellent feedback on the proposal and implementation, but without the help for the following individuals, it would not have been possible to get DTrace into PostgreSQL.

Gavin Sherry - Gavin was the first person in the community to help us identify locations in PostgreSQL to insert probes. We initially wanted to create a demo, and with a short notice, Gavin made himself available to talk with us.

Tom Lane - At the PostgerSQL Anniverary Summit, Tom helped verify the probe locations and corrected a few of them and provide excellent feedback on how the framework should be implemented.

Peter Eisentraut - Peter stepped up to help incorporate DTrace into PostgreSQL build system. His help was invaluable in getting DTrace in 8.2 before code freeze.

Angelo Rajadurai - Angelo is a DTrace guru in MDE, and he was a great help in getting me up to speed with adding user-level probes and DTrace in general.

Adam Leventhal and Bryan Cantrill (The DTrace creators) - For making themselves available to answer questions and provide feedback.




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