Friday, August 29, 2014

DB2 10 for z/OS, 1 MB Page Frames, and the Number 6656

I recently received an interesting message from a mainframe DB2 person I've known for a long time. At his DB2 10 for z/OS site, 1 MB page frames had recently been made available on the System z servers, and my friend moved to leverage this resource via page-fixed DB2 buffer pools. Nothing unusual there. What's interesting is the information he saw in the output of -DISPLAY BUFFERPOOL commands subsequently entered for the page-fixed pools. I'll get to that strange-looking data in a moment, but first I'll provide a bit of background information for level-set purposes.

Fixing the buffers of a buffer pool in real storage so that they can't be paged out to auxiliary storage is an option that was introduced with DB2 V8 for z/OS -- I first blogged on the topic about six years ago, in an entry I posted to the blog that I maintained while working as an independent DB2 consultant (prior to rejoining IBM in 2010). In DB2 V8 and V9 environments, specifying PGFIX(YES) for a high-I/O buffer pool would improve a system's processing efficiency by reducing the CPU cost of I/O operations associated with the pool. That same benefit -- fewer CPU cycles consumed in reading and writing DB2 table space and index pages from and to disk (and from and to group buffer pools in a data sharing system) -- is realized as well in DB2 10 (and 11) systems when buffer pools are page-fixed, but another opportunity for CPU savings is added: starting with DB2 10, a buffer pool defined with PGFIX(YES) will be backed by 1 MB real storage page frames, versus traditional 4 KB frames, if 1 MB frames are available in the z/OS LPAR. The 1 MB page frames enhance performance by making translation of virtual storage addresses to real storage addresses more efficient (this because the larger frames lead to more "hits" in the translation lookaside buffer). How are 1 MB page frames made available in a z/OS LPAR? That's done by way of the LFAREA parameter in the IEASYSxx member of SYS1.PARMLIB (the ability to have some portion of a z/OS LPAR's memory resource managed in 1 MB frames was introduced with the IBM z10 mainframes and z/OS 1.10). With this exploitation by DB2 of 1 MB page frames, PGFIX(YES) can boost CPU efficiency even for buffer pools that have little in the way of I/O activity.

Back now to my friend the DB2 systems programmer. As I mentioned, he issued -DISPLAY BUFFERPOOL commands for his page-fixed pools to get some information on their allocation. Now, this wasn't the standard-issue -DISPLAY BUFFERPOOL command. What was issued was a particular (and not extensively documented) form of the command, shown below with BP0 used as an example:


When -DISPLAY BUFFERPOOL is used with that SERVICE(4) option in a DB2 10 environment (the story is different for DB2 11 -- I'll get to that later), the output will include DSNB999I messages showing the allocation of buffers in a pool with regard to the type of real storage frame backing the buffers (don't bother looking for DSNB999I in the DB2 10 Messages manual, because it's not there). For a 4K buffer pool with VPSIZE(9000) and PGFIX(YES), this is what my DB2 friend saw in the DSNB999I part of the -DISPLAY BUFFERPOOL output:

DSNB999I  -DBP1 4K PAGES 2344
DSNB999I  -DBP1 1M PAGES 6656

That 6656 number looked odd to the DB2 systems programmer, and to me, as well. Why would DB2 allocate 6656 of the pool's buffers with 1 MB page frames, and the other 2344 buffers (a little over 9 MB) with 4 KB frames? It wasn't because of a shortage of 1 MB page frames -- there were plenty of those available in the LPAR (if there are not enough 1 MB frames to fully back a PGFIX(YES) buffer pool, DB2 10 or 11 will use what 1 MB frames there are and then use 4 KB frames for the rest of the pool's buffers). My friend looked at -DISPLAY BUFFERPOOL output for his other page-fixed pools, and he saw, in each case, that the number of buffers allocated with 1 MB page frames was a multiple of 6656 (and this was true regardless of the size of a pool's buffers -- he saw the same thing for 4K and  32K pools). The buffers beyond the multiple of 6656 for a page-fixed pool were allocated with 4 KB frames. So, a 4K pool with VPSIZE(30000) and PGFIX(YES) was seen to have 26,624 buffers (4 X 6656) allocated with 1 MB frames and 3376 buffers allocated with 4 KB frames. A 32K pool with VPSIZE(10000) and PGFIX(YES) had 6656 buffers allocated with 1 MB frames and 3344 buffers allocated with 4 KB frames.

In researching this strange-looking situation, the DB2 systems programmer and I came across a relatively recent APAR, PI12512, and that (plus a little more digging on my part) gave us our answer. DB2 uses control blocks to track buffers in a pool, and DB2 10 initially allocated control blocks for a PGFIX(YES) pool in such a way that it made sense to allocate the control blocks in 1 MB chunks and to back with 1 MB page frames the number of buffers in the pool covered by these 1 MB chunks of control blocks. It so happens that 1 MB of buffer pool control blocks can cover 6656 buffers, and that's why buffers in a PGFIX(YES) pool were allocated in multiples of 6656 with 1 MB page frames, with any buffers left over (i.e., any beyond the largest multiple of 6656 that was less than a pool's VPSIZE) backed by 4 KB frames.

When the fix for APAR PI12512 is applied to a DB2 10 subsystem, things work differently: buffers in a PGFIX(YES) pool are allocated in 1 MB chunks with 1 MB page frames, and any buffers left over, after as many 1 MB frames as possible have been filled, are allocated to 4 KB frames. The text of APAR PI12512 points up one advantage of this new arrangement: page-fixed buffer pools with a VPSIZE value of less than 6656 can be backed by 1 MB page frames. That is indeed a good thing, but there is a benefit for larger pools, as well. That benefit is best illustrated by example. Recall that my DB2 systems programmer friend saw, for a 4K pool with VPSIZE(9000) and PGFIX(YES), that 6656 buffers (about 74% of the total) were allocated with 1 MB page frames and 2344 buffers were allocated with 4 KB frames. If the fix for APAR PI12512 were applied to the DB2 system (and if the pool were deallocated and reallocated, which of course would be a consequence of the DB2 stop and restart required to activate the maintenance), for that same buffer pool one would expect to see 8960 buffers (99.5% of the total -- 35 chunks of 256 four kilobyte buffers) allocated with 1 MB frames, leaving only 40 buffers to be allocated with 4 KB frames. The greater the percentage of a page-fixed pool's buffers that can be allocated with 1 MB page frames, the better, in terms of performance, because the efficiency gained through decreased translation lookaside buffer misses will be maximized.

I should point out here that the change in the allocation of control blocks for page-fixed buffer pools that made possible the 1 MB page frame benefit maximization delivered by APAR PI12512 was accomplished through the fix for another APAR, PM85944 (and that fix was ultimately provided via APAR PM90486).

OK, now for the DB2 11 perspective. APAR PI12512 is a DB2 10-only APAR (and that's also the case for APARs PM85944 and PM90486). DB2 11 already has the functionality that these APARs provided for DB2 10; furthermore, with DB2 11 you don't have to issue -DISPLAY BUFFERPOOL with that secret-code-looking SERVICE(4) option to see the allocation of a pool's buffers with regard to different page frame sizes. And, with DB2 11 you won't see DSNB999I (the message you can't find in the DB2 for z/OS Messages manual) in -DISPLAY BUFFERPOOL output. Issue the simple and familiar command shown below in a DB2 11 environment (and here I'm using BP1 as an example):


and you'll see that the output includes something that looks like this:


The "preferred frame size" indicates what was specified for the FRAMESIZE option of -ALTER BUFFERPOOL for the pool. That's a new option introduced with DB2 11 for z/OS. For a PGFIX(NO) pool, DB2 will automatically allocate the pool's buffers with 4 KB page frames. If there are 1 MB page frames available in a z/OS LPAR, and PGFIX(YES) is specified for a buffer pool, DB2 will automatically prefer 1 MB page frames for the pool (even so, in that case you might want to consider an explicit specification of FRAMESIZE(1M), just to be intentional about it). You can't use 2 GB frames for a buffer pool (new with DB2 11) unless the LFAREA specification provides some 2 GB frames in the z/OS LPAR, and the pool is defined with PGFIX(YES), and DB2 is running in a zEC12 server (you probably wouldn't see much, if any, performance benefit with a FRAMESIZE(2G) specification for a pool -- that configuration option is likely to be more advantageous when z/OS LPAR memory sizes are considerably larger than what is typically seen these days).

Below the "preferred frame size" line of the DSNB546I message (and that message can be found in the DB2 11 for z/OS Messages manual), you see information on the actual allocation of a pool's buffers with respect to different page frame sizes. There is one DSNB546I message for each different page frame size used in the allocation of a pool's buffers (in my example above, the buffer pool had a VPSIZE of 4000, and all of the pool's buffers were allocated with 4 KB page frames).

There you have it. That 6656 number (or a multiple thereof) will continue to show up in -DISPLAY BUFFERPOOL(BPn) SERVICE(4) output in DB2 10 for z/OS systems (for page-fixed pools in z/OS LPARs that have 1 MB page frames), until the fix for APAR PI12512 is applied. It won't show up in DB2 11 environments. Someday, we DB2 old-timers will reminisce about the strange days of buffers being allocated with 1 MB page frames in 6656-buffer chunks, same as we look back now on old and odd-seeming things like index subpages and hiperpools in expanded storage. And young whippersnappers will roll their eyes.

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