Thread

1. Unexpected page allocation behavior on insert-only tables

   Michael Renner <michael.renner@amd.co.at> — 2010-05-15T23:53:49Z

   While preparing a replication test setup with 9.0beta1 I noticed strange 
   page allocation patterns which Andrew Gierth found interesting enough to 
   report here.
   
   I've written a simple tool to generate traffic on a database [1], which 
   did about 30 TX/inserts per second to a table. Upon inspecting the data 
   in the table, I noticed the expected grouping of tuples which came from 
   a single backend to matching pages [2]. The strange part was that the 
   pages weren't completely filled but the backends seemed to jump 
   arbitrarily from one page to the next [3]. For the table in question 
   this resulted in about 10% wasted space.
   
   After issuing a VACUUM on the table the free space map got updated (or 
   initialized?) and the backends used the remaining space in the pages, 
   though the spurious page allocation continued.
   
   
   best regards,
   Michael
   
   [1] https://workbench.amd.co.at/hg/pgworkshop/file/dc5ab49c99bb/pgexerciser
   
   [2] E.g.:
   
   (0,1) TX1
   (0,2) TX5
   (0,3) TX7
   ..
   (1,1) TX2
   (1,2) TX6
   (1,3) TX9
   
   etc.
   
   [3] http://nopaste.narf.at/show/55/
   Optimal usage seems to be 136 tuples per page for the table in question.
   
   

2. Re: Unexpected page allocation behavior on insert-only tables

   Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-16T00:16:47Z

   Michael Renner <michael.renner@amd.co.at> writes:
   > I've written a simple tool to generate traffic on a database [1], which 
   > did about 30 TX/inserts per second to a table. Upon inspecting the data 
   > in the table, I noticed the expected grouping of tuples which came from 
   > a single backend to matching pages [2]. The strange part was that the 
   > pages weren't completely filled but the backends seemed to jump 
   > arbitrarily from one page to the next [3]. For the table in question 
   > this resulted in about 10% wasted space.
   
   Which table would that be?  The trigger-driven updates to "auction",
   in particular, would certainly guarantee some amount of "wasted" space.
   
   			regards, tom lane
   
   

3. Re: Unexpected page allocation behavior on insert-only tables

   Michael Renner <michael.renner@amd.co.at> — 2010-05-16T00:24:36Z

   On 16.05.2010 02:16, Tom Lane wrote:
   > Michael Renner<michael.renner@amd.co.at>  writes:
   >> I've written a simple tool to generate traffic on a database [1], which
   >> did about 30 TX/inserts per second to a table. Upon inspecting the data
   >> in the table, I noticed the expected grouping of tuples which came from
   >> a single backend to matching pages [2]. The strange part was that the
   >> pages weren't completely filled but the backends seemed to jump
   >> arbitrarily from one page to the next [3]. For the table in question
   >> this resulted in about 10% wasted space.
   >
   > Which table would that be?  The trigger-driven updates to "auction",
   > in particular, would certainly guarantee some amount of "wasted" space.
   
   Yeah, the auction table receives heavy updates and gets vacuumed regularly.
   
   The behavior I showed was for the "bid" table, which only gets inserts 
   (and triggers the updates for the auction table).
   
   best regards,
   Michael
   
   

4. Re: Unexpected page allocation behavior on insert-only tables

   Alvaro Herrera <alvherre@alvh.no-ip.org> — 2010-05-17T20:14:45Z

   Excerpts from Michael Renner's message of sáb may 15 20:24:36 -0400 2010:
   > On 16.05.2010 02:16, Tom Lane wrote:
   > > Michael Renner<michael.renner@amd.co.at>  writes:
   > >> I've written a simple tool to generate traffic on a database [1], which
   > >> did about 30 TX/inserts per second to a table. Upon inspecting the data
   > >> in the table, I noticed the expected grouping of tuples which came from
   > >> a single backend to matching pages [2]. The strange part was that the
   > >> pages weren't completely filled but the backends seemed to jump
   > >> arbitrarily from one page to the next [3]. For the table in question
   > >> this resulted in about 10% wasted space.
   > >
   > > Which table would that be?  The trigger-driven updates to "auction",
   > > in particular, would certainly guarantee some amount of "wasted" space.
   > 
   > Yeah, the auction table receives heavy updates and gets vacuumed regularly.
   > 
   > The behavior I showed was for the "bid" table, which only gets inserts 
   > (and triggers the updates for the auction table).
   
   I think this may be related to the smgr_targblock stuff; if the relcache
   entry gets invalidated at the wrong time for whatever reason, the
   "current page" could be abandoned in favor of extending the rel.  This
   has changed since 8.4, but a quick perusal suggests that it should be
   less likely on 9.0 than 8.4 but maybe there's something weird going on.
   -- 
   
   

5. Re: Unexpected page allocation behavior on insert-only tables

   Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-31T02:42:25Z

   Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
   > Excerpts from Michael Renner's message of sáb may 15 20:24:36 -0400 2010:
   >>> I've written a simple tool to generate traffic on a database [1], which
   >>> did about 30 TX/inserts per second to a table. Upon inspecting the data
   >>> in the table, I noticed the expected grouping of tuples which came from
   >>> a single backend to matching pages [2]. The strange part was that the
   >>> pages weren't completely filled but the backends seemed to jump
   >>> arbitrarily from one page to the next [3]. For the table in question
   >>> this resulted in about 10% wasted space.
   
   > I think this may be related to the smgr_targblock stuff; if the relcache
   > entry gets invalidated at the wrong time for whatever reason, the
   > "current page" could be abandoned in favor of extending the rel.  This
   > has changed since 8.4, but a quick perusal suggests that it should be
   > less likely on 9.0 than 8.4 but maybe there's something weird going on.
   
   I found time to try this example finally.  The behavior that I see in
   HEAD is even worse than Michael describes: there is room for 136 rows
   per block in the bid table, but most blocks have only a few rows.  The
   distribution after letting the exerciser run for 500 bids or so is
   typically like this:
   
     #rows  block#
       136  0
         6  1
         5  2
         4  3
         3  4
         5  5
         3  6
         1  7
         4  8
         4  9
       136  10
         6  11
         7  12
         9  13
         9  14
         7  15
         9  16
         7  17
         8  18
         5  19
       136  20
         2  21
         4  22
         4  23
         3  24
         5  25
         3  26
         4  27
         3  28
         2  29
         1  30
   
   Examining the insertion timestamps and bidder numbers (client process
   IDs), and correlating this with logged autovacuum activity, makes it
   pretty clear what is going on.  See the logic in
   RelationGetBufferForTuple, and note that at no time do we have any FSM
   data for the bid table:
   
   1. Initially, all backends will decide to insert into block 0.  They do
   so until the block is full.
   
   2. At that point, each active backend individually decides it needs to
   extend the relation.  They each create a new block and start inserting
   into that one, each carefully not telling anyone else about the block
   so as to avoid block-level insertion contention.  In the above diagram,
   blocks 1-9 are each created by a different backend and the rows inserted
   into it come (mostly?) from just one backend.  Block 10's first few rows
   also come from the one backend that created it, but it doesn't manage to
   fill the block entirely before ...
   
   3. After awhile, autovacuum notices all the insert activity and kicks
   off an autoanalyze on the bid table.  When committed, this forces a
   relcache flush for each other backend's relcache entry for "bid".
   In particular, the smgr targblock gets reset.
   
   4. Now, all the backends again decide to try to insert into the last
   available block.  So everybody jams into the partly-filled block 10,
   until it gets filled.
   
   5. Lather, rinse, repeat.  Since there are exactly 10 active clients
   (by default) in this test program, the repeat distance is exactly 10
   blocks.
   
   The obvious thing to do about this would be to not reset targblock
   on receipt of a relcache flush event, but we can *not* do that in the
   general case.  The reason that that gets reset is so that it's not
   left pointing to a no-longer-existent block after a VACUUM truncation.
   Maybe we could develop a way to distinguish truncation events from
   others, but right now the sinval signaling mechanism can't do that.
   This looks like there might be sufficient grounds to do something,
   though.
   
   Attached exhibits: contents of relevant columns of the bid table
   and postmaster log entries for autovacuum actions during the run.
   
   			regards, tom lane
   
   

6. Re: Unexpected page allocation behavior on insert-only tables

   Robert Haas <robertmhaas@gmail.com> — 2010-05-31T03:44:02Z

   On Sun, May 30, 2010 at 10:42 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
   > pretty clear what is going on.  See the logic in
   > RelationGetBufferForTuple, and note that at no time do we have any FSM
   > data for the bid table:
   
   Is this because, in the absence of updates or deletes, we never vacuum it?
   
   > 4. Now, all the backends again decide to try to insert into the last
   > available block.  So everybody jams into the partly-filled block 10,
   > until it gets filled.
   
   Would it be (a) feasible and (b) useful to inject some entropy into this step?
   
   -- 
   Robert Haas
   EnterpriseDB: http://www.enterprisedb.com
   The Enterprise Postgres Company
   
   

7. Re: Unexpected page allocation behavior on insert-only tables

   Takahiro Itagaki <itagaki.takahiro@oss.ntt.co.jp> — 2010-05-31T03:46:40Z

   Tom Lane <tgl@sss.pgh.pa.us> wrote:
   
   > 3. After awhile, autovacuum notices all the insert activity and kicks
   > off an autoanalyze on the bid table.  When committed, this forces a
   > relcache flush for each other backend's relcache entry for "bid".
   > In particular, the smgr targblock gets reset.
   > 
   > 4. Now, all the backends again decide to try to insert into the last
   > available block.  So everybody jams into the partly-filled block 10,
   > until it gets filled.
   
   The autovacuum process runs only analyze, but does not run vacuum at 3
   because the workload is insert-only. Partially filled pages are never
   tracked by freespace map. We could re-run an autovacuum if we saw the
   report from the autoanalyze that says the table is low-density,
   but the additional vacuum might be overhead in other cases.
   
   > The obvious thing to do about this would be to not reset targblock
   > on receipt of a relcache flush event
   
   Even if we don't reset targblock, can we solve the issue when clients
   connect and disconnect for each insert? New backends only check the end
   of the table, and extend it as same as this case. If we are worrying
   about the worst caase, we might need to track newly added pages with
   freespace map. Of course we can ignore the case because frequent
   connections and disconnections should be always avoided.
   
   Regards,
   ---
   Takahiro Itagaki
   NTT Open Source Software Center
   
   
   
   

8. Re: Unexpected page allocation behavior on insert-only tables

   Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-31T04:03:59Z

   Robert Haas <robertmhaas@gmail.com> writes:
   > On Sun, May 30, 2010 at 10:42 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
   >> pretty clear what is going on. See the logic in
   >> RelationGetBufferForTuple, and note that at no time do we have any FSM
   >> data for the bid table:
   
   > Is this because, in the absence of updates or deletes, we never vacuum it?
   
   Right.
   
   >> 4. Now, all the backends again decide to try to insert into the last
   >> available block. So everybody jams into the partly-filled block 10,
   >> until it gets filled.
   
   > Would it be (a) feasible and (b) useful to inject some entropy into this step?
   
   Maybe, but at least in this case, the insert rate is not fast enough
   that contention for the block is worth worrying about.  IMO this isn't
   the part of the cycle that needs fixed.
   
   I guess another path to a fix might be to allow the backends to record
   new pages in the FSM immediately at creation.  That might result in more
   insert contention, but it'd avoid losing track of the free space
   permanently, which is what is happening here (unless something happens
   to cause a vacuum).  One reason the current code doesn't do that is that
   the old in-memory FSM couldn't efficiently support retail insertion of
   single-page data, but the new FSM code hasn't got a problem with that.
   
   			regards, tom lane
   
   

9. Re: Unexpected page allocation behavior on insert-only tables

   Greg Stark <gsstark@mit.edu> — 2010-05-31T05:09:49Z

   On Mon, May 31, 2010 at 3:42 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
   > note that at no time do we have any FSM
   > data for the bid table:
   >
   >
   > 3. After awhile, autovacuum notices all the insert activity and kicks
   > off an autoanalyze on the bid table.  When committed, this forces a
   > relcache flush for each other backend's relcache entry for "bid".
   > In particular, the smgr targblock gets reset.
   
   This is an analyze-only scan? Why does analyze need to issue a
   relcache flush? Maybe we only need to issue one for an actual vacuum
   which would also populate the fsm?
   
   
   -- 
   greg
   
   

10. Re: Unexpected page allocation behavior on insert-only tables

    Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-31T05:23:37Z

    Greg Stark <gsstark@mit.edu> writes:
    > This is an analyze-only scan? Why does analyze need to issue a
    > relcache flush?
    
    Directly: to cause other backends to pick up the updated pg_class row
    (with new relpages/reltuples data).
    
    Indirectly: to cause cached plans for the rel to be invalidated,
    so that they can get replanned with updated pg_statistic entries.
    
    So we can't just not have a relcache flush here.  However, we
    might be able to decouple targblock reset from the rest of it.
    In particular, now that there's a distinction between smgr flush
    and relcache flush, maybe we could associate targblock reset with
    smgr flush (only) and arrange to not flush the smgr level during
    ANALYZE --- basically, smgr flush would only be needed when truncating
    or reassigning the relfilenode.  I think this might work out nicely but
    haven't chased the details.
    
    			regards, tom lane
    
    

11. Re: Unexpected page allocation behavior on insert-only tables

    Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-31T20:47:39Z

    I wrote:
    > In particular, now that there's a distinction between smgr flush
    > and relcache flush, maybe we could associate targblock reset with
    > smgr flush (only) and arrange to not flush the smgr level during
    > ANALYZE --- basically, smgr flush would only be needed when truncating
    > or reassigning the relfilenode.  I think this might work out nicely but
    > haven't chased the details.
    
    I looked into that a bit more and decided that it'd be a ticklish
    change: the coupling between relcache and smgr cache is pretty tight,
    and there just isn't any provision for having an smgr cache entry live
    longer than its owning relcache entry.  Even if we could fix it to
    work reliably, this approach does nothing for the case where a backend
    actually exits after filling just part of a new page, as noted by
    Takahiro-san.
    
    The next most promising fix is to have RelationGetBufferForTuple tell
    the FSM about the new page immediately on creation.  I made a draft
    patch for that (attached).  It fixes Michael's scenario nicely ---
    all pages get filled completely --- and a simple test with pgbench
    didn't reveal any obvious change in performance.  However there is
    clear *potential* for performance loss, due to both the extra FSM
    access and the potential for increased contention because of multiple
    backends piling into the same new page.  So it would be good to do
    some real performance testing on insert-heavy scenarios before we
    consider applying this.  Any volunteers?
    
    Note: patch is against HEAD but should work in 8.4, if you reverse out
    the use of the rd_targblock access macros.
    
    			regards, tom lane
    
    

12. Re: Unexpected page allocation behavior on insert-only tables

    Bruce Momjian <bruce@momjian.us> — 2011-02-05T03:10:48Z

    Tom Lane wrote:
    > I wrote:
    > > In particular, now that there's a distinction between smgr flush
    > > and relcache flush, maybe we could associate targblock reset with
    > > smgr flush (only) and arrange to not flush the smgr level during
    > > ANALYZE --- basically, smgr flush would only be needed when truncating
    > > or reassigning the relfilenode.  I think this might work out nicely but
    > > haven't chased the details.
    > 
    > I looked into that a bit more and decided that it'd be a ticklish
    > change: the coupling between relcache and smgr cache is pretty tight,
    > and there just isn't any provision for having an smgr cache entry live
    > longer than its owning relcache entry.  Even if we could fix it to
    > work reliably, this approach does nothing for the case where a backend
    > actually exits after filling just part of a new page, as noted by
    > Takahiro-san.
    > 
    > The next most promising fix is to have RelationGetBufferForTuple tell
    > the FSM about the new page immediately on creation.  I made a draft
    > patch for that (attached).  It fixes Michael's scenario nicely ---
    > all pages get filled completely --- and a simple test with pgbench
    > didn't reveal any obvious change in performance.  However there is
    > clear *potential* for performance loss, due to both the extra FSM
    > access and the potential for increased contention because of multiple
    > backends piling into the same new page.  So it would be good to do
    > some real performance testing on insert-heavy scenarios before we
    > consider applying this.  Any volunteers?
    > 
    > Note: patch is against HEAD but should work in 8.4, if you reverse out
    > the use of the rd_targblock access macros.
    
    Is this something we want to address or should I just add it to the
    TODO?
    
    -- 
      Bruce Momjian  <bruce@momjian.us>        http://momjian.us
      EnterpriseDB                             http://enterprisedb.com
    
      + It's impossible for everything to be true. +
    
    

13. Re: Unexpected page allocation behavior on insert-only tables

    Bruce Momjian <bruce@momjian.us> — 2011-02-17T03:22:25Z

    Tom Lane wrote:
    > I wrote:
    > > In particular, now that there's a distinction between smgr flush
    > > and relcache flush, maybe we could associate targblock reset with
    > > smgr flush (only) and arrange to not flush the smgr level during
    > > ANALYZE --- basically, smgr flush would only be needed when truncating
    > > or reassigning the relfilenode.  I think this might work out nicely but
    > > haven't chased the details.
    > 
    > I looked into that a bit more and decided that it'd be a ticklish
    > change: the coupling between relcache and smgr cache is pretty tight,
    > and there just isn't any provision for having an smgr cache entry live
    > longer than its owning relcache entry.  Even if we could fix it to
    > work reliably, this approach does nothing for the case where a backend
    > actually exits after filling just part of a new page, as noted by
    > Takahiro-san.
    > 
    > The next most promising fix is to have RelationGetBufferForTuple tell
    > the FSM about the new page immediately on creation.  I made a draft
    > patch for that (attached).  It fixes Michael's scenario nicely ---
    > all pages get filled completely --- and a simple test with pgbench
    > didn't reveal any obvious change in performance.  However there is
    > clear *potential* for performance loss, due to both the extra FSM
    > access and the potential for increased contention because of multiple
    > backends piling into the same new page.  So it would be good to do
    > some real performance testing on insert-heavy scenarios before we
    > consider applying this.  Any volunteers?
    
    I have added this TODO:
    
    	Allow concurrent inserts to use recently created pages rather than
    	creating new ones
    	
    	    * http://archives.postgresql.org/pgsql-hackers/2010-05/msg00853.php 
    
    -- 
      Bruce Momjian  <bruce@momjian.us>        http://momjian.us
      EnterpriseDB                             http://enterprisedb.com
    
      + It's impossible for everything to be true. +