ignite-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Roman Kondakov <kondako...@mail.ru.INVALID>
Subject Re: Page eviction for in-memory mode with enabled MVCC
Date Mon, 17 Dec 2018 11:32:19 GMT
Ivan,

probably you are right. The main usage of in-memory cache with allowed 
evictions is a caching layer for third party stores. And the fact that 
page eviction is not a transactional process by the nature, forces user 
to use workarounds (i.e. explicit locking) to prevent evictions of the 
hot data. This workaround is quite a heavyweight solution as well. Using 
explicit locking (select for update, etc.) for each read request may 
lead to the increased number of aborted transactions due to the need to 
obtain an exclusive lock for each key we read. The repeatable read 
semantics for MVCC caches with evictions will result to the performance 
drop, which makes useless this application of MVCC caches.

Perhaps, we should prohibit MVCC caches creations in regions with 
configured eviction policy, as you proposed?

Igor, Vladimir, what do you think?


-- 
Kind Regards
Roman Kondakov

On 17.12.2018 8:53, Павлухин Иван wrote:
> Roman,
>
> Thank you for pointing out usage as an in-memory cache. I will try to
> describe how do I see the use case.
>
> First of all our MVCC caches provides transactions. And a user will
> choose MVCC if his workflow is transactional. If a use case is a
> caching layer then some backing storage is assumed. But we have not
> yet well integrated support for 3rd party persistence [1]. And I think
> that it is better to cover whole track in complex.
>
> Of course there might be another valid use cases which I am not aware
> of. Please point me out if you have one in mind.
>
> [1] https://apacheignite.readme.io/docs/3rd-party-store
>
> 2018-12-14 18:40 GMT+03:00, Seliverstov Igor <gvvinblade@gmail.com>:
>> Roman,
>>
>> I would prefer first option.
>>
>> The fact user uses MVCC says he needs some more strict guaranties which
>> cannot meet in other modes.
>> I would rollback both txs in case we cannot provide such guaranties.
>>
>> Regards,
>> Igor
>>
>> пт, 14 дек. 2018 г. в 15:36, Roman Kondakov <kondakov87@mail.ru.invalid>:
>>
>>> Vladimir,
>>>
>>> I was thinking about your proposal to not evict locked and recent (the
>>> transaction that created the record is still active) entries from the
>>> cache. Let's imagine next situation: we have almost full memory and two
>>> transactions:
>>>
>>> 1. txA: "SELECT * FOR UPDATE"
>>>
>>> 2. txB: "INSERT ...many keys here..."
>>>
>>> In this case txA locks all entries in the cache, and therefore we cannot
>>> evict any of them. If then txB is trying to add a lot of data, it lead
>>> us to the OOM situation, which user is trying to avoid using cache
>>> evictions.
>>>
>>> I see two ways how to deal with this issue:
>>>
>>> 1. Allow OOM in MVCC caches with configured evictions and warn user
>>> about it in the docs.
>>>
>>> 2. Give up with the repeatable read guaranties in case of evictions for
>>> MVCC caches and warn users about it in the documentation.
>>>
>>> Second variant looks better for me because user may not expect OOM when
>>> he has configured eviction policy for cache.
>>>
>>> What do you think?
>>>
>>>
>>> --
>>> Kind Regards
>>> Roman Kondakov
>>>
>>> On 13.12.2018 22:33, Vladimir Ozerov wrote:
>>>> It's hard to believe that entries are not locked on backups, because we
>>>> wrtite data right away. Even if it so, it should be very easy to fix -
>>> just
>>>> do not evict and entry if it was created or deleted by currently active
>>>> transaction.
>>>>
>>>> On Thu, Dec 13, 2018 at 10:28 PM Roman Kondakov
>>> <kondakov87@mail.ru.invalid>
>>>> wrote:
>>>>
>>>>> Vladimir,
>>>>>
>>>>> we do not lock entries on backups when MVCC is enabled and therefore
>>>>> we
>>>>> don't avoid entry eviction from backup by locking. So, your first
>>>>> scenario with primary stop is still relevant.
>>>>>
>>>>>
>>>>> --
>>>>> Kind Regards
>>>>> Roman Kondakov
>>>>>
>>>>> On 13.12.2018 22:14, Vladimir Ozerov wrote:
>>>>>> No, I mean that we should think about what kind of guarantees it
>>>>> possible.
>>>>>> My proposal was to prevent evictions of locked entries. This way
we
>>>>>> can
>>>>> say
>>>>>> users: "if you want true REPEATABLE_READ when evictions are enabled,
>>> then
>>>>>> make sure to lock entries on every access". This effectively means
>>>>>> that
>>>>> all
>>>>>> SELECT's should be replaced with "SELECT FOR UPDATE".
>>>>>>
>>>>>> On Thu, Dec 13, 2018 at 10:09 PM Roman Kondakov
>>>>> <kondakov87@mail.ru.invalid>
>>>>>> wrote:
>>>>>>
>>>>>>> Vladimir,
>>>>>>>
>>>>>>> correct me please if i misunderstood your thought. So, if eviction
>>>>>>> is
>>>>>>> not about a consistency at all, we may evict keys in any way
because
>>>>>>> broken repeatable read semantics is not the biggest problem here.
>>>>>>> But
>>> we
>>>>>>> should add some notes about it to user documentation. Right?
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Kind Regards
>>>>>>> Roman Kondakov
>>>>>>>
>>>>>>> On 13.12.2018 17:45, Vladimir Ozerov wrote:
>>>>>>>> Roman,
>>>>>>>>
>>>>>>>> I would start with the fact that eviction can never be consistent
>>>>> unless
>>>>>>> it
>>>>>>>> utilizes atomic broadcast protocol, which is not the case
for
>>>>>>>> Ignite.
>>>>> In
>>>>>>>> Ignite entries on node are evicted independently.
>>>>>>>>
>>>>>>>> So you may easily get into situation like this:
>>>>>>>> 1) Start a cache with 1 backup and FULL_SYNC mode
>>>>>>>> 2) Put a key to primary node
>>>>>>>> 3) Stop primary
>>>>>>>> 4) Try reading from new primary and get null because key
was
>>>>>>>> evicted
>>>>>>>> concurrently
>>>>>>>>
>>>>>>>> Or:
>>>>>>>> 1) Start a transaction in PESSIMISTIC/READ_COMMITTED mode
>>>>>>>> 2) Read a key, get value
>>>>>>>> 3) Read the same key again, get null
>>>>>>>>
>>>>>>>> So in reality the choice is not between consistent and inconsistent
>>>>>>>> behavior, but rather about degree of inconsistency. Any solution
is
>>>>>>>> possible as long as we can explain it to the user. E.g. "do
not
>>> evict a
>>>>>>> key
>>>>>>>> if it is either write-locked".
>>>>>>>>
>>>>>>>>
>>>>>>>> On Thu, Dec 13, 2018 at 5:19 PM Vladimir Ozerov <
>>> vozerov@gridgain.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> Andrey,
>>>>>>>>>
>>>>>>>>> We will not be able to cache the whole data set locally,
as it
>>>>>>> potentially
>>>>>>>>> lead to OOME. We will have this only as an option and
only for
>>> non-SQL
>>>>>>>>> updates. Thus, similar semantics is not possible.
>>>>>>>>>
>>>>>>>>> On Thu, Dec 13, 2018 at 4:56 PM Andrey Mashenkov <
>>>>>>>>> andrey.mashenkov@gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> Roman,
>>>>>>>>>>
>>>>>>>>>> We have a ticket to improve repeatable_read mode
[1] via caching
>>>>>>> entries
>>>>>>>>>> locally.
>>>>>>>>>> This should make mvcc transaction repeatable_read
semantic
>>>>>>>>>> similar
>>> to
>>>>>>>>>> non-mvcc Txs
>>>>>>>>>> and allow us to implement eviction in correct way.
>>>>>>>>>>
>>>>>>>>>> Another way is to introduce mvcc shared (read) entry
locks and
>>> evict
>>>>>>> only
>>>>>>>>>> entries if no one hold any lock on it,
>>>>>>>>>> but this looks tricky and error prone as your first
one as it may
>>>>> lead
>>>>>>> to
>>>>>>>>>> eviction policy unexpected behavior,
>>>>>>>>>> e.g some versions can be visible while others - no
(evicted).
>>>>>>>>>>
>>>>>>>>>> [1] https://issues.apache.org/jira/browse/IGNITE-7371
>>>>>>>>>>
>>>>>>>>>> On Thu, Dec 13, 2018 at 4:34 PM Ilya Kasnacheev <
>>>>>>>>>> ilya.kasnacheev@gmail.com>
>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>> Hello!
>>>>>>>>>>>
>>>>>>>>>>> Is it possible to 'touch' entries read by MVCC
transactions to
>>>>> ensure
>>>>>>>>>> that
>>>>>>>>>>> they are considered recent and therefore are
almost never
>>>>>>>>>>> targeted
>>>>> by
>>>>>>>>>>> eviction?
>>>>>>>>>>>
>>>>>>>>>>> This is 1) with benefits.
>>>>>>>>>>>
>>>>>>>>>>> Regards,
>>>>>>>>>>> --
>>>>>>>>>>> Ilya Kasnacheev
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> чт, 13 дек. 2018 г. в 16:22, Roman Kondakov
>>>>>>> <kondakov87@mail.ru.invalid
>>>>>>>>>>> :
>>>>>>>>>>>
>>>>>>>>>>>> Hi igniters,
>>>>>>>>>>>>
>>>>>>>>>>>> I need your advice with the following issue.
When in-memory
>>>>>>>>>>>> cache
>>>>>>>>>>>> reaches it's memory limit, some data may
be purged to avoid OOM
>>>>>>> error.
>>>>>>>>>>>> This process is described in [1]. For MVCC
caches this eviction
>>> may
>>>>>>>>>>>> break repeatable read semantics. For example,
if transaction
>>> reads
>>>>>>> key
>>>>>>>>>>>> before eviction, this key is visible for
it. But if key is
>>> evicted
>>>>>>>>>> some
>>>>>>>>>>>> time later, this key become invisible to
anyone, including our
>>>>>>>>>>>> transaction, which means broken repeatable
read semantics.
>>>>>>>>>>>>
>>>>>>>>>>>> Now we see the following solutions of this
problem:
>>>>>>>>>>>>
>>>>>>>>>>>> 1. Ignore broken repeatable read semantics.
If cache is set to
>>>>> allow
>>>>>>>>>>>> data eviction, it may lose it's data. This
means that there is
>>>>>>>>>>>> no
>>>>>>>>>>>> valuable information stored in cache and
occasional repeatable
>>> read
>>>>>>>>>>>> violations can be tolerated.
>>>>>>>>>>>>
>>>>>>>>>>>> 2. Prohibit eviction of MVCC caches at all.
For example, stop
>>>>> writing
>>>>>>>>>> to
>>>>>>>>>>>> caches and throw an appropriate exception
in the case when
>>>>>>>>>>>> there
>>> is
>>>>>>> no
>>>>>>>>>>>> free space in page memory. Before this exception
Ignite should
>>>>>>>>>>>> do
>>>>>>> it's
>>>>>>>>>>>> best to avoid this situation, for example,
evict all non-mvcc
>>>>> caches
>>>>>>>>>> and
>>>>>>>>>>>> run full vacuum to free as much space as
possible.
>>>>>>>>>>>>
>>>>>>>>>>>> First approach is bad because it leads to
cache consistency
>>>>>>> violation.
>>>>>>>>>>>> Second approach is bad because it's behavior
may be unexpected
>>>>>>>>>>>> to
>>>>>>> user
>>>>>>>>>>>> if he has set an eviction policy for cache,
but instead of
>>> eviction
>>>>>>>>>>>> Ignite trying to avoid it as much as possible.
>>>>>>>>>>>>
>>>>>>>>>>>> IMO first approach looks better - it is much
simpler to
>>>>>>>>>>>> implement
>>>>> and
>>>>>>>>>>>> met user expectations in all points except
possible repeatable
>>> read
>>>>>>>>>>>> violations.
>>>>>>>>>>>>
>>>>>>>>>>>> What do you think?
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> [1] https://apacheignite.readme.io/docs/evictions
>>>>>>>>>>>>
>>>>>>>>>>>> --
>>>>>>>>>>>> Kind Regards
>>>>>>>>>>>> Roman Kondakov
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>> Best regards,
>>>>>>>>>> Andrey V. Mashenkov
>>>>>>>>>>
>

Mime
View raw message