1. 28 Sep, 2016 2 commits
  2. 14 Aug, 2016 1 commit
    • Kirill Smelkov's avatar
      bigfile/zodb/ZBlk1: Don't miss to deactivate/free internal .chunktab buckets in loadblkdata() · 542917d1
      Kirill Smelkov authored
      13c0c17c (bigfile/zodb: Format #1 which is optimized for small changes)
      used BTree to organize ZBlk1 block's chunks and for loadblkdata() added
      "TODO we are missing to free internal BTree structures on data load".
      #3 besides other
      things showed that even when we deactivate ZData objects, we are still
      keeping them as ghosts occupying memory and the same for IOBucket
      This all happens because there is no proper way to deactivate whole
      btree - including internal buckets objects. And since internal buckets
      are not deactivated, they stay in picklecache and thus hold a reference
      to ZData objects and ZData objects in turn, even if explicitly
      deactivated, stay in memory.
      We can fix this all via implementing whole-btree deactivation procedure.
      To do so we need to iterate over all btree buckets recursively, but
      unfortunately there is no BTree API to access/iterate btree's buckets.
      We can however still get reference to first top-level buckets via
      gc.get_referents(btree) and then scan buckets further without hacks.
      gc.get_referents(btree) is a hack, but
      - it works in O(1)  (we only get pointers from btree, not scanning all
        gcable objects and deducing them)
      - it works reliable if we filter out non-interesting objects.
      So in the end it works.
      Before the patch loading more and more ZBlk1 data with objgraph
      instrumentation was showing itself like
          #                                    Nobj        δ
          wendelin.bigfile.file_zodb.ZData     7168      +512
          BTrees.IOBTree.IOBucket               238       +17
          BTrees.IOBTree.IOBTree                 14        +1
      and after this patch we now have
          BTrees.IOBTree.IOBTree                 14        +1
      we cannot remove that "IOBTree + 1", since ZBlk1 is holding direct
      reference on it (via .chunktab) and we have to keep ZBlk1 live with
      ._v_zfile and ._v_zblk set for invalidation to work. "+1 IOBtree" is
      however small - 144 bytes per 2M (= 0.006%) so we can neglect that the
      same way we neglect keeping ZBlk1 staying live for each block.
  3. 14 Jul, 2016 4 commits
    • Kirill Smelkov's avatar
      wendelin.core v0.7 · f7803634
      Kirill Smelkov authored
    • Kirill Smelkov's avatar
      bigfile/virtmem: usleep() needs unistd.h · d9d6409f
      Kirill Smelkov authored
      The following started to appear after recent gcc upgrade on my host:
      bigfile/virtmem.c: In function `vma_on_pagefault':
      bigfile/virtmem.c:696:9: warning: implicit declaration of function `usleep' [-Wimplicit-function-declaration]
               usleep(10000);  // XXX with 1000 uslepp still busywaits
    • Kirill Smelkov's avatar
      setup: Propagate only setuptools, not whole sys.path · 8f624957
      Kirill Smelkov authored
      This updates and fixes 487e5226 (setup: specify setuptools location
      explicitly when calling make.) to use @kazuhiko original idea to
      propagate only setuptools location. The reason is - when propagating
      whole sys.path things break under tox tests:
      ---- 8< ----
      ========================================================================== test session starts ===========================================================================
      platform linux2 -- Python 2.7.12, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
      rootdir: /home/kirr/src/wendelin/release/wendelin.core, inifile:
      collected 33 items
      bigarray/tests/test_arrayzodb.py .......
      bigarray/tests/test_basic.py ........
      bigfile/tests/test_basic.py ....
      bigfile/tests/test_filefile.py .
      bigfile/tests/test_filezodb.py ........
      bigfile/tests/test_thread.py ....
      lib/tests/test_calc.py .
      ======================================================================= 33 passed in 14.14 seconds =======================================================================
      x86_64-linux-gnu-gcc -pthread -g -Wall -D_GNU_SOURCE -std=gnu99 -fplan9-extensions -Wno-declaration-after-statement -Wno-error=declaration-after-statement  -Iinclude -I3rdparty/ccan -I3rdparty/include   bigfile/tests/tfault.c lib/bug.c lib/utils.c 3rdparty/ccan/ccan/tap/tap.c  -o bigfile/tests/tfault.t
      t/tfault-run bigfile/tests/tfault.t faultr on_pagefault
      ok 1 - !pagefault_init()
      Fatal Python error: Py_Initialize: Unable to get the locale encoding
        File ".../wendelin.core/.tox/py27-ZODB3-zblk0-fs-numpy110/lib/python2.7/encodings/__init__.py", line 123
          raise CodecRegistryError,\
      SyntaxError: invalid syntax
      Current thread 0x00007f9b80024780 (most recent call first):
      t/tfault-run: line 28: 21521 Аварийный останов         (core dumped) gdb -q -batch $tfault core > core.info
      E: can't gdb(core)
      Makefile:189: ошибка выполнения рецепта для цели «faultr.tfault»
      make: *** [faultr.tfault] Ошибка 1
      rm bigfile/tests/test_virtmem.t bigfile/tests/test_ram.t bigfile/tests/tfault.t bigfile/tests/test_pagemap.t
      error: Failed to execute `make test`
      ERROR: InvocationError: '.../wendelin.core/.tox/py27-ZODB3-zblk0-fs-numpy110/bin/python setup.py test'
      ________________________________________________________________________________ summary _________________________________________________________________________________
      ERROR:   py27-ZODB3-zblk0-fs-numpy110: commands failed
      ---- 8< ----
      What happens here is:
      - gdb is used in automated tests
      - gdb is linked with libpython3.5
      - tox is currently running tests with python27
      - setup.py sets PYTHONPATH to whole path from python27
      - gdb, upon starting, initializes python runtime, which tries to load
        py27 modules under py35 -> oops.
      So propagating only setuptools location practically solves the issue for
      now, but still there is potential risk that in future, there will be
      other modules put in the same location as setuptools (location is parent
      dir of a module/package) which python tries to load on startup and which
      might be incompatible between 2 & 3.
      Thus not setting PYTHONPATH at all - e.g. creating environments
      virtualenv way - would be better, but buildout fundamentally works the
      other way. We can not also use -c 'buildout sys.path hack' in $PYTHON
      itself, as $PYTHON is used in general way inside Makefile.
      So let the hack with setuptools location in PYTHONPATH stay there until
      it practically works.
    • Kazuhiko's avatar
      setup: specify setuptools location explicitly when calling make. · 487e5226
      Kazuhiko authored
      If wendelin.core is built under buildout and setuptools exists only in
      buildout environment, 'python setup.py' called inside make will fail
      without this change.
          Building 'wendelin.core'
          Running easy_install:
          "/path/to/python2.7" "-c" "import sys; sys.path[0:0] = ['/path/to/setuptools-19.6.2-py2.7.egg']; from setuptools.command.easy_install import main; main()"...
          <<< setup.py: os.system('make %s PYTHON="%s"' % (target, sys.executable))
          make[1]: Leaving directory '/tmp/xxx/wendelin.core-0.6/3rdparty/ccan'
          /path/to/python2.7 setup.py ll_build_ext --inplace
          Traceback (most recent call last):
            File "setup.py", line 18, in <module>
              from setuptools import setup, Extension, Command, find_packages
          ImportError: No module named setuptools
          Makefile:40: recipe for target 'bigfile/_bigfile.so' failed
          make: *** [bigfile/_bigfile.so] Error 1
          error: Setup script exited with error: Failed to execute `make all`
      [ @kirr: as a solution we are propagating whole python path via make.
        This should cover both setuptools case, and any other future potential
        "preloaded-by-buildout" egg. ]
      /reviewed-on !1
  4. 06 Jul, 2016 1 commit
    • Kirill Smelkov's avatar
      Fix build for Python 3.5 · e6beab19
      Kirill Smelkov authored
      @kazuhiko reports that wendelin.core build is currently broken on Python 3.5.
      Indeed it was:
          In file included from bigfile/_bigfile.c:37:0:
          ./include/wendelin/compat_py2.h: In function ‘_PyThreadState_UncheckedGetx’:
          ./include/wendelin/compat_py2.h:66:28: warning: implicit declaration of function ‘_Py_atomic_load_relaxed’ [-Wimplicit-function-declaration]
               return (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current);
          ./include/wendelin/compat_py2.h:66:53: error: ‘_PyThreadState_Current’ undeclared (first use in this function)
               return (PyThreadState*)_Py_atomic_load_relaxed(&_PyThreadState_Current);
          ./include/wendelin/compat_py2.h:66:53: note: each undeclared identifier is reported only once for each function it appears in
          ./include/wendelin/compat_py2.h:67:1: warning: control reaches end of non-void function [-Wreturn-type]
      The story here is that in 3.5 they decided to remove direct access to
      _PyThreadState_Current and atomic implementations - because that might
      semantically conflict with other headers implementing atomics - and
      provide only access by function.
      Starting from Python 3.5.2rc1 the function to get current thread state
      without asserting it is !NULL - _PyThreadState_UncheckedGet() - was added:
      so for those python versions we can directly use it.
      After the fix wendelin.core tox tests pass under all python2.7, python3.4 and python3.5.
      More context here:
      Fixes: #1
  5. 01 Jul, 2016 2 commits
  6. 27 Jun, 2016 2 commits
  7. 24 Jun, 2016 1 commit
    • Kirill Smelkov's avatar
      bigfile/pagemap: Fix non-leaf page iteration · ee9bcd00
      Kirill Smelkov authored
      Since the beginning of pagemap (45af76e6 "bigfile/pagemap: specialized
      {} uint64 -> void * mapping") we had a bug sitting in
      __pagemap_for_each_leaftab() (non-leaf iterating logic behind
      After entry to stack-down was found, we did not updated tailv[l]
      accordingly. Thus if there are non-adjacent entries an entry could be
      e.g. emitted many times:
           l 3  __down 0x7f79da1ee000
           tailv[4]: 0x7f79da1ee000
            -> tailv[4] 0x7f79da1ee000  __down 0x7f79da1ed000
           l 4  __down 0x7f79da1ed000
           tailv[5]: 0x7f79da1ed000
           h 5  l 5  leaftab: 0x7f79da1ed000      <--
            lvl 5  idx 169  page 0x55aa
          ok 9 - pagemap_for_each(0) == 21930
           l 5  __down (nil)
           tailv[4]: 0x7f79da1ee008
            -> tailv[4] 0x7f79da1ee008  __down 0x7f79da1ed000
           l 4  __down 0x7f79da1ed000
           tailv[5]: 0x7f79da1ed000
           h 5  l 5  leaftab: 0x7f79da1ed000      <--
            lvl 5  idx 169  page 0x55aa
          not ok 10 - pagemap_for_each(1) == 140724106500272
      And many-time-emitted entries are not only incorrect, but can also lead
      to not-handled segmentation faults in e.g. fileh_close():
          /* drop all pages (dirty or not) associated with this fileh */
          pagemap_for_each(page, &fileh->pagemap) {
              /* it's an error to close fileh to mapping of which an access is
               * currently being done in another thread */
              BUG_ON(page->state == PAGE_LOADING);
              list_del(&page->lru);                           <-- HERE
              bzero(page, sizeof(*page)); /* just in case */
      ( because after first bzero of a page, the page is all 0 bytes including
        page->lru{.next,.prev} so on the second time when the same page is
        emitted by pagemap_for_each, list_del(&page->lru) will try to set
        page->lru.next = ... which will segfault. )
      So fix it by properly updating tailv[l] while we scan/iterate current level.
      This applies only to non-leaf pagemap levels, as leaf level is scanned
      with separate loop in pagemap_for_each. That's why we probably did not
      noticed this earlier - up until now our usual workloads was to change
      data in adjacent batches and that means adjacent pages.
      Though today @Tyagov was playing with wendelin.core in some other way and
      it uncovered the bug.
  8. 13 Jun, 2016 5 commits
  9. 12 Jun, 2016 1 commit
    • Kirill Smelkov's avatar
      3rdparty/ccan: Update · 89d099af
      Kirill Smelkov authored
      Just update to latest CCAN for it to be a fresh one.
      Throught the modules we use there are no real updates, just a fix for
      one warning in array_size.
  10. 20 Apr, 2016 1 commit
    • Kirill Smelkov's avatar
      bigfile/zodb: Don't write ZBlk to DB if it was not changed · 6773fda0
      Kirill Smelkov authored
      For ZBlk1 we already compare ZData content about whether it was changed
      compared to data already stored to DB, and do not store it twice if data
      is the same.
      However ZBlk itself is always marked as changed, if corresponding memory
      page was dirtied. This results in transactions like
      Trans #33915309 tid=03b6944919befeee time=2016-04-17 22:01:06.034237 offset=140320105842
          status=' ' user='...' description='...'
        # ... other parts, but no ZData here
        data #2 oid=000000000026fc4c size=79 class=wendelin.bigfile.file_zodb.ZBlk1
      where ZBlk1 is committed the same without necessity.
      NOTE we cannot avoid committing ZBlk in all cases, because it is used to signal
          other DB clients that a ZBlk needs to be invalidated and this way associated
          fileh pages are invalidated too.
          This cannot work via ZData, because ZData don't have back-pointer to
          ZBlk1 or to corresponding zfile.
  11. 05 Apr, 2016 1 commit
    • Kirill Smelkov's avatar
      Fix develop install for setuptools >= 19.4 · 2ce96a76
      Kirill Smelkov authored
      Starting from setuptools 19.4, more concrete from the following commit:
      setuptools sorts namespaced packages .__path__ to be in sync with sys.path .
      That however breaks for wendelin.core used from in-tree or installed in
      development mode, because we are doing tricks in top-level import redirector
      (see e870781d "Top-level in-tree import redirector"):
          (z+numpy.v2)kirr@teco:~/tmp/trashme/wendelin.core$ python -c 'import wendelin'
          Traceback (most recent call last):
            File "<string>", line 1, in <module>
            File "wendelin.py", line 39, in <module>
            File "/home/kirr/src/wendelin/venv/z+numpy.v2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2081, in declare_namespace
              _handle_ns(packageName, path_item)
            File "/home/kirr/src/wendelin/venv/z+numpy.v2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2026, in _handle_ns
              _rebuild_mod_path(path, packageName, module)
            File "/home/kirr/src/wendelin/venv/z+numpy.v2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2050, in _rebuild_mod_path
            File "/home/kirr/src/wendelin/venv/z+numpy.v2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2045, in position_in_sys_path
              return sys_path.index(_normalize_cached(os.sep.join(parts)))
          ValueError: '/home/kirr/tmp/trashme' is not in list
      Here wendelin.py added /home/kirr/tmp/trashme/wendelin.core to .__path__ and
      setuptools' _handle_ns() wants to order that dir's parent in correspondence
      with sys.path, but parent path is not there - oops.
      We can workaround the problem, by first not initializing .__path__ and letting
      fully handle and initialize it, and only after it is done we make the
      correction for wendelin modules located not under .../wendelin.core/wendelin/
      but under .../wendelin.core/ .
      Importing was tested to work with the fix with both setuptools 20.6.7 and older
      setuptools 17.1.1, i.e. here we should not be breaking backward compatibility.
      /reported-by @tatuya, @Camata, @Tyagov
      /reviewed-on kirr/wendelin.core!1
  12. 18 Dec, 2015 1 commit
    • Kirill Smelkov's avatar
      ZBigArray: Compatibility fix to read arrays from DB that were previously saved without order info · 2ca0f076
      Kirill Smelkov authored
      Commit ab9ca2df (bigarray: Add support for FORTRAN ordering) added
      ability to define array order, but there I made a mistake of not caring
      about how previously-saved to DB arrays would be read back.
      The thing is BigArray gained new data member ._order which is
      automatically saved to DB thanks to ZBigArray inheriting from
      Persistent; on load-from-db path we just read object state from DB,
      which for ZBigArray is dict, and restore object attributes from it.
      But for previously-saved data, obviously, there is no 'order' entry and thus
      this way restored objects are restored not in full to current code expectations
      and it can boom e.g. this way:
        Module wendelin.bigarray, line 190, in resize
          self._init0(new_shape, self.dtype, order=self._order)
        AttributeError: 'ZBigArray' object has no attribute '_order'
      Solution to fix is: on restore-from-DB path, see if a data member is not
      present on restored object, and if it has default value in BigArray set it to
      ( code to get function defaults is from
        http://stackoverflow.com/questions/12627118/get-a-function-arguments-default-value )
      /cc @Tyagov, @klaus
  13. 15 Dec, 2015 6 commits
    • Kirill Smelkov's avatar
      tox: Bump NEO to 1.6 · 18b40b18
      Kirill Smelkov authored
    • Kirill Smelkov's avatar
      bigfile/virtmem: Do loadblk() with virtmem lock released · f49c11a3
      Kirill Smelkov authored
      loadblk() calls are potentially slow and external code that serve the cal can
      take other locks in addition to virtmem lock taken by virtmem subsystem. If
      that "other locks" are also taken before external code calls e.g.
      fileh_invalidate_page() in different codepath a deadlock can happen, e.g.
            T1                  T2
            page-access         invalidation-from-server received
            V -> loadblk
                                Z   <- ClientStorage.invalidateTransaction()
            Z -> zeo.load
                                V   <- fileh_invalidate_page
      The solution to avoid deadlock is to call loadblk() with virtmem lock released
      and upon loadblk() completion recheck virtmem data structures carefully.
      To make that happen:
      - new page state is introduces:
          PAGE_LOADING                (file content loading is  in progress)
      - virtmem releases virt_lock before calling loadblk() when serving pagefault
      - because loading is now done with virtmem lock released, now:
      1. After loading completes we need to recheck fileh/vma data structures
         The recheck is done in full - vma_on_pagefault() just asks its driver (see
         VM_RETRY and VM_HANDLED codes) to retry handling the fault completely. This
         should work as the freshly loaded page was just inserted into fileh->pagemap
         and should be found there in the cache on next lookup.
         On the other hand this also works correctly, if there was concurrent change
         - e.g. vma was unmapped while we were loading the data - in that case the
         fault will be also processed correctly - but loaded data will stay in
         fileh->pagemap (and if not used will be evicted as not-needed
         eventually by RAM reclaim).
      2. Similar to retrying mechanism is used for cases when two threads
         concurrently access the same page and would both try to load corresponding
         block - only one thread issues the actual loadblk() and another waits for load
         to complete with polling and VM_RETRY.
      3. To correctly invalidate loading-in-progress pages another new page state
         is introduced:
          PAGE_LOADING_INVALIDATED    (file content loading was in progress
                                       while request to invalidate the page came in)
         which fileh_invalidate_page() uses to propagate invalidation message to
         loadblk() caller.
      4. Blocks loading can now happen in parallel with other block loading and
         other virtmem operations - e.g. invalidation. For such cases tests are added
         to test_thread.py
      5. virtmem lock now becomes just regular lock, instead of being previously
         For virtmem lock to be recursive was needed for cases, when code under
         loadblk() could trigger other virtmem calls, e.g. due to GC and calling
         another VMA dtor that would want to lock virtmem, but virtmem lock was
         already held.
         This is no longer needed.
      6. To catch double faults we now cannot use just on static variable
         in_on_pagefault. That variable thus becomes thread-local.
      7. Old test in test_thread to "test that access vs access don't overlap" no
         longer holds true - and is thus removed.
      /cc @Tyagov, @klaus
    • Kirill Smelkov's avatar
      bigfile/virtmem: Factor functionality to unlock/retake GIL into own functions · 0231a65d
      Kirill Smelkov authored
      Previously we were doing virt_lock() / virt_unlock() which automatically
      were making sure to unlock GIL before locking virtmem, and to restore
      GIL state to previous after virtmem lock happened. virt_unlock() was
      unlocking just the virtmem lock without touching GIL at all - that works
      because the running code would eventually release GIL as python
      regularly does so to allowing multiple threads to run.
      In the next patch however, we'll need to wait for in-progress-loading
      page to complete, and that wait has to be done with GIL released (so
      other python threads could run), and for doing so we'll need
      functionality to make sure GIL is unlocked and retake it back, not tied
      to virt_lock().
      So factor it out.
    • Kirill Smelkov's avatar
      bigfile/tests: move NotifyChannel to test_thread.py · 99cd1f03
      Kirill Smelkov authored
      NotifyChannel was introduced in c7c01ce4 (bigfile/zodb: ZODB.Connection
      can migrate between threads on close/open and we have to care) to test
      thread interaction specific to ZODB.
      We'll however need NotifyChannel to do more threading test of virtmem
      core, and this way the proper place for NotifyChannel is test_thread.py
      Move it.
    • Kirill Smelkov's avatar
      bigfile/virtmem: Remove obsolete XXX about locking · 81bf620c
      Kirill Smelkov authored
      Both comments are from the beginning - from 9a293c2d (bigfile/virtmem:
      Userspace Virtual Memory Manager) - but d53271b9 patch (bigfile/virtmem:
      Big Virtmem lock) missed to update them.
    • Kirill Smelkov's avatar
      bigfile: Plug memory leak in ramh_close() · 997ebacd
      Kirill Smelkov authored
      No one was freeing RAMH structure itself, and thus ASAN reports e.g.:
      ==15935==ERROR: LeakSanitizer: detected memory leaks
      Direct leak of 32 byte(s) in 1 object(s) allocated from:
          #0 0x7f29c89f1001 in __interceptor_calloc (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x94001)
          #1 0x401da1 in zalloc include/wendelin/utils.h:65
          #2 0x408128 in shmfs_ramh_open bigfile/tests/../ram_shmfs.c:202
          #3 0x407611 in ramh_open bigfile/tests/../ram.c:81
          #4 0x402560 in fileh_open bigfile/tests/../virtmem.c:131
          #5 0x427ca1 in test_pagefault_savestate bigfile/tests/test_virtmem.c:1022
          #6 0x4281ba in main bigfile/tests/test_virtmem.c:1061
          #7 0x7f29c83b8b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)
      NOTE similar leak remains open in ram_close(), but it is a bit involved
      to fix and the effort will be removed anyway after we switch to kernel
      virtual memory manager. Besides ramh are opened and closed all the time
      and ram only once.
  14. 05 Nov, 2015 1 commit
    • Kirill Smelkov's avatar
      bigfile/ram_shmfs: Warn users explicitly if fallocate() is not supported · e8c05a22
      Kirill Smelkov authored
      Currently wendelin.core does not work on e.g. Debian 7, because that
      distro has too old kernel without support for fallocate on tmpfs.
      But the diagnostics of failure is not clear and looks like just being
      out of memory:
          bigfile/tests/../virtmem.c:845 OOM      BUG!
      what happens in fact is that
          - virtmem tries to allocate a page -> calls shmfs_alloc_page(),
          - fallocate() fails with "operation not supported" error code
          - virtmem sees this as page allocation failure,
          - tries to reclaim pages,
          - but there are no allocated pages at all -> OOM
      Detect whether fallocate() error is operational error, or simply
      "fallocate not supported" and if latter, report to user. Now it looks
          bigfile/tests/../ram_shmfs.c:129 shmfs_alloc_page WARN: fallocate() not supported
          bigfile/tests/../virtmem.c:845 OOM      BUG!
      /cc @Tyagov
  15. 02 Nov, 2015 1 commit
  16. 02 Oct, 2015 9 commits
  17. 30 Sep, 2015 1 commit
    • Kirill Smelkov's avatar
      bigfile/zodb: Teach ZBlk1 not to waste memory on memory-page -> DB path · 1bf0cf31
      Kirill Smelkov authored
      ZBlk* objects are intermediate ZODB object in between data stored in
      ZODB and memory pages managed by virtmem. As such, after they do their
      job to either load data from DB to memory, or store from memory to DB,
      it is not needed to keep them alive with duplicate content thus only
      wasting memory.
      ZBlk0 cares about this detail via "deactivating" ._v_blkdata in
      loadblkdata() and __getstate__() prologues.
      ZBlk1 did the same for load path in loadblkdata() prologue, but for
      .__getstate__() it was not directly possible, because for ZBlk1 the
      state is IOBTree, not one non-persistent object, and thus it first needs
      to be processed by ZODB together with its subobjects on its way to
      storage and only then all they deactivated.
      So 13c0c17c (bigfile/zodb: Format #1 which is optimized for small
      changes) only put TODO for memory-page -> DB path about not wasting
      memory this way.
      But the problem is relatively easy to solve:
          - we can deactivate ZData objects (leaf objects in ZBlk1.chunktab
            btree) by hooking into ZData.__getstate__() prologue;
          - we also need to care to deactivate chunks right away, which
            setblkdata() loaded to compare .data and found them to be not
      This way we do not waste memory keeping intermediate ZData objects alive
      with the same content as memory page after commit.
      /cc @Tyagov