Re: reiser4: discard implementation, pass 2: allocation issues

[Edward Shishkin <edward.shishkin@gmail.com>]

On 06/16/2014 01:00 PM, Ivan Shapovalov wrote:
> On Monday 16 June 2014 at 11:24:39, Edward Shishkin wrote:	
>> On 06/16/2014 07:03 AM, Ivan Shapovalov wrote:
>>> On Monday 16 June 2014 at 02:14:49, Edward Shishkin wrote:	
>>>> On 06/15/2014 11:58 PM, Ivan Shapovalov wrote:
>>>>> On Sunday 15 June 2014 at 23:49:59, Edward Shishkin wrote:	
>>>>>> On 06/15/2014 08:07 PM, Ivan Shapovalov wrote:
>>>>>>> On Sunday 15 June 2014 at 19:36:05, Edward Shishkin wrote:	
>>>>>>>> On 06/13/2014 10:28 PM, Ivan Shapovalov wrote:
>>>>>>>>> Here is my "analysis" of what happens in reiser4 during a transaction's
>>>>>>>>> lifetime wrt. block allocation and deallocation.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> THE EFFECTS (SEMANTICS) OF RELATED FUNCTIONS
>>>>>>>>> reiser4_alloc_blocks_bitmap(): allocates in WORKING BITMAP
>>>>>>>> Yes.
>>>>>>>>
>>>>>>>>> reiser4_dealloc_blocks_bitmap(!BA_DEFER): deallocates from WORKING BITMAP
>>>>>>>>> reiser4_dealloc_blocks_bitmap(BA_DEFER): stores to ->delete_set
>>>>>>>> This is correct for the middle-level allocator (without suffix "bitmap").
>>>>>>>> The low-level one frees blocks only in WORKING BITMAP.
>>>>>>> Yes, this was a wording mistake. I've noticed it shortly after sending...
>>>>>>>
>>>>>>>>> reiser4_pre_commit_hook_bitmap(): allocates all relocated nodes in COMMIT BITMAP
>>>>>>>> "Relocated" is bad term here: nodes with new data also get the flag
>>>>>>>> JNODE_RELOC. So, I would rather say, that it applies freshly allocated
>>>>>>>> nodes of the atom to COMMIT BITMAP.
>>>>>>>>
>>>>>>>>
>>>>>>>>>                                        deallocates ->delete_set from COMMIT BITMAP
>>>>>>>> applies the atom's delete_set to COMMIT BITMAP
>>>>>>> I've meant exactly this.
>>>>>>>
>>>>>>>>> reiser4_post_commit_hook(): deallocates ->delete_set using !BA_DEFER
>>>>>>>>>                                                         (i. e. from WORKING BITMAP)
>>>>>>>> applies the atom's delete_set to WORKING BITMAP.
>>>>>>> ditto
>>>>>>>
>>>>>>>> I would also mention a function of the middle-level block allocator
>>>>>>>> reiser4_alloc_blocks(): allocates blocks in WORKING  BITMAP.
>>>>>>> Yes, sure.
>>>>>>>
>>>>>>>
>>>>>>>> Note that the middle-level block allocator (block_alloc.c) actually
>>>>>>>> manipulates
>>>>>>>> with abstract space maps. Currently in reiser4 they are represented only by
>>>>>>>> bitmaps (plugin/space/bitmap.c). We can also implement another
>>>>>>>> representation -
>>>>>>>> extent tree (like in XFS). I don't see any needs for now, though.
>>>>>>> Extent trees seem to be interesting.. They claim that they are more efficient -
>>>>>>> is the difference that huge?
>>>>>>>
>>>>>>>>> TIMELINE OF ALLOCATIONS FOR "USUAL" NODES, AND TIMELINE OF TRANSACTION COMMIT
>>>>>>>>> - nodes are allocated using reiser4_alloc_blocks() and setting JNODE_RELOC,
>>>>>>>>>        so WORKING BITMAP ensures that two nodes cannot get the same block;
>>>>>>>>> - nodes are deallocated using reiser4_dealloc_blocks(BA_DEFER),
>>>>>>>>>        so their deallocation is not immediately reflected in WORKING BITMAP;
>>>>>>>>> (the relocate set is written here)
>>>>>>>>> - reiser4_pre_commit_hook_bitmap() uses 1) JNODE_RELOC flag and 2) ->delete_set
>>>>>>>>>        to convey effective bitmap changes into COMMIT BITMAP;
>>>>>>>>> (the journal and overwrite set are written here)
>>>>>>>>> - reiser4_post_commit_hook() uses ->delete_set to convey deallocations
>>>>>>>>>        from step 2 to WORKING BITMAP.
>>>>>>>>> (the discard happens here)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> TIMELINE OF ALLOCATIONS FOR WANDERED JOURNAL BLOCKS
>>>>>>>>> - at commit time, blocks are allocated using reiser4_alloc_blocks(), so they
>>>>>>>>>        are allocated in WORKING BITMAP and do not interfere with any "usual" blocks;
>>>>>>>>> - after writing wandered blocks, they are deallocated using
>>>>>>>>>        reiser4_dealloc_blocks(!BA_DEFER), i. e. from the WORKING BITMAP.
>>>>>>>> So, the system of working and commit bitmaps plus the delete set seems
>>>>>>>> to be redundant? I think this is because of performance reasons: block
>>>>>>>> allocation is critical thing...
>>>>>>> Seems like it is -- for performance and simplicity (== robustness).
>>>>>>>
>>>>>>>>> CONCLUSION
>>>>>>>>> At possible transaction replay time, journal blocks are not allocated in any
>>>>>>>>> of the bitmaps. However, because the journal is read and replayed before a
>>>>>>>>> transaction has a chance to commit, this fact does not matter.
>>>>>>>>> What matters is that wandered journal blocks never hit COMMIT BITMAP.
>>>>>>>>>
>>>>>>>>> So, if I've got all this correct (which I highly doubt), the disk space leak
>>>>>>>>> (as you pointed it out) does not exist.
>>>>>>>> It seems, you are right..
>>>>>>>>
>>>>>>>>> What exists is a rather different problem with my idea of "log every
>>>>>>>>> deallocated block". Current implementation logs every block regardless of
>>>>>>>>> BA_DEFER flag presence or absence, so non-wandered blocks are logged twice.
>>>>>>>>>
>>>>>>>>> We could just use ->delete_set, but we would lose wandered blocks then.
>>>>>>>>> Or we could only log !BA_DEFER requests, which would do the right thing
>>>>>>>>> (wandered blocks + deallocations from reiser4_post_commit_hook()), but
>>>>>>>>> the reasoning behind this decision would not be obvious for a casual
>>>>>>>>> code reader.
>>>>>>>> I think that a good comment will save the situation..
>>>>>>>>
>>>>>>>>> Or we could log only wandered blocks (in addition to ->delete_set)
>>>>>>>>> at discard time, but this is messy and requires us to merge the discard log
>>>>>>>>> with ->delete_set at discard time.
>>>>>>>> what is the difference with the previous "we could.."?
>>>>>>> The previous option is to log all !BA_DEFER requests in addition to
>>>>>>> ->delete_set, so those two block sets would be partially overlapping.
>>>>>> Hmm.. Are they really overlapped?
>>>>>>
>>>>>> reiser4_dealloc_blocks() looks like the following:
>>>>>> {
>>>>>> if (flags & BA_DEFER)
>>>>>>             log in the delete_set;
>>>>>> else
>>>>>>             deallocate in working bitmap
>>>>>> ...
>>>>>> }
>>>>>>
>>>>>> so (!BA_DEFER) requests and the delete_set look disjoint,
>>>>>> or, I missed something?
>>>>> You're right, I've misread the code. I was under impression that
>>>>> reiser4_post_commit_hook() internally calls reiser4_dealloc_blocks(!BA_DEFER).
>>>>>
>>>>> So the second option (which I prefer) is to log all sa_dealloc_blocks() calls.
>>>>> That is, effectively, ->delete_set plus wandered journal block deallocations.
>>>> Yes, I think to do something like this:
>>>>
>>>> reiser4_dealloc_blocks()
>>>> {
>>>> if (flags & BA_DEFER) {
>>>>            ...
>>>>            } else {
>>>>            ...
>>>>            if (discard is turned on)
>>>>                    /* we'll want to discard these blocks, which are not to
>>>> be represented
>>>>                        in the COMMIT SPACE MAP, so store them in a
>>>> separate list */
>>>>                    do {
>>>>                            blocknr_set_add_extent(atom,
>>>> &atom->delete_set_for_wander, &bsep, start, len);
>>>>                            ...
>>>>                    } while (ret == -E_REPEAT);
>>>> sa_dealloc_blocks(reiser4_get_space_allocator(ctx->super), *start, *len);
>>>>            ...
>>>> }
>>>>
>>>> Then join the sets (->delete_set, and ->delete_set_for_wander) at
>>>> discard time
>>>> (should be pretty cheap operation). insert a check at every atoms
>>>> fusion, that
>>>> ->delete_set_for_wander of both atoms are empty).
>>> Hm. IIRC, ultimately we wanted to use rb-trees for storing discard block sets,
>>> but this means that we'll need to convert two blocknr_sets into an rb-tree
>>> at discard time, which pretty defeats their purpose in our context... Or do you
>>> say that blocknr_set itself should be made rb-tree?
>> I didn't know about the delete sets. They are not optional and lists are
>> better for them (as they don't require sorting). So I suggest to confine
>> with lists for now, and may be to try the fast-merge trees in the future
>> (if there is a great desire).
> The discard algorithm still needs its input to be sorted, and blocknr_set is
> inherently unordered.
>
> So there are two options:
> - log into ->delete_set and ->delete_set_for_wander (as you've proposed),
> then merge them into a sorted list at discard time;
> - log into ->discard_set (just like it is now), just fix what to log.
>
> What do you think?


Are the blocknr sets comfortable?
Say, can we organize the discard process via the blocknr_set_iterator()?
If yes, then let's do everything via the blocknr sets (i.e. let's 
implement the
first option). Add needed operations to blocknrset.c, the discard iteration
actor to discard.c, etc.