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From Alex Plehanov <plehanov.a...@gmail.com>
Subject Re: [DISCUSS] Page replacement improvement
Date Mon, 23 Nov 2020 07:31:00 GMT
Nikolay,

> Can you list disadvantages of the Random-LRU?
Already described this in previous letters: "To find the next page for
replacement in the best case we scan 5 pages, in the worst case we can scan
all data region segment. Also, due to random nature, it's not very
effective in predicting the right page for replacement to minimize the
page-fault rate."

> Is this scenario correct?
No, page replacement will start right after step 1. When you put data to
cache bigger than available RAM. The performance will be the same in steps
2 and 4.
To benchmark different page replacement algorithm, just the first two steps
from your scenario are enough.
To benchmark batch page replacement we should make some data colder than
another. For example, the scenario where batch page replacement can be
useful:
1. Put data bigger than available RAM.
2. Work with some set of keys for some period of time.
3. Work with another set of keys and benchmark performance.
4. Trigger batch page replacement in parallel to replace cold data from
step 2.
Without step 4 on step 3, there will be more page replacements, and I
suppose performance with step 4 will be better.



пн, 23 нояб. 2020 г. в 09:48, Nikolay Izhikov <nizhikov@apache.org>:

> Hello, Alex.
>
> > Perhaps we can implement a special benchmark for this case with manually
> triggered "batch page replacement" using yardstick (but I'm not sure if
> ducktape can help here).
>
> I think we should carefully describe the issues with the current approach
> and then we can choose right tool to benchmark improvements.
> You said:
>
> >  we use Random-LRU algorithm … it has many disadvantages and affects
> performance very much when replacement is started
>
> Can you list disadvantages of the Random-LRU?
>
> AFAIU the first benchmark should be:
>
> 1. Start cluster with persistence and put data bigger then available RAM
> to it.
> 2. Measure performance of the queries that selects one entry from the
> cache.
> 3. Make some queries that will iterate throw all data - `SELECT SUM(x)
> FROM t` or something similar.
> 4. Repeat step 2. in this time performance of random queries should be
> lower due to the page replacement.
>
> Is this scenario correct?
>
> > 23 нояб. 2020 г., в 09:12, Alex Plehanov <plehanov.alex@gmail.com>
> написал(а):
> >
> > Nikolay, Zhenya,
> >
> > Benchmark from IGNITE-13034 is fully synthetic, it makes random puts
> > uniformly. It can be used to compare different page replacement
> algorithms
> > (random-LRU vs segmented-LRU), but it is totally inapplicable to
> benchmark
> > batch page replacement.
> > Perhaps we can implement a special benchmark for this case with manually
> > triggered "batch page replacement" using yardstick (but I'm not sure
> > if ducktape can help here).
> >
> >> I understand case you described, but who will pull the switch ? Human,
> > artificial intelligence ?
> > As I described before, we can implement both manual and automatic "batch
> > page replacement" triggering. For automatic triggering, there is no
> > artificial intelligence needed, just several conditions with reasonable
> > thresholds. Automatic triggering also can be disabled by default.
> >
> > пт, 20 нояб. 2020 г. в 13:32, Zhenya Stanilovsky
> <arzamas123@mail.ru.invalid
> >> :
> >
> >>
> >>
> >>
> >>
> >>
> >>
> >>> Zhenya,
> >>>
> >>>> Alexey, we already have changes that partially fixes this issue [1]
> >>> IGNITE-13086 it's a minor improvement. We still have major problems
> with
> >>> our page replacement algorithm (slow page selection and non-optimal
> >>> page-fault rate). I think changing from random 5 pages to 7 will make
> >>> things even worse (it's better for page-fault rate, but page selection
> >> will
> >>> be slower).
> >> All this words above need to be proven, i hope. + 1 with Nikolay, we
> need
> >> correct reproduces or some graphs from 2.9 ver.
> >>
> >>>
> >>>> This approach still not applicable for real life
> >>> Why do you think batch replacement is not applicable for real-life? It
> can
> >>> be applied for workloads, where some big amount of data periodically
> used,
> >>> but not very often. For example, when OLAP request over historical data
> >>> raised pages to page-memory, and after such request this data is not
> >> needed
> >>> for a long time. Or when OLTP transactions mostly add new data and
> process
> >>> recent data but rarely touch historical data. In these cases with the
> >>> current approach, we will enter "page replacement mode" after some
> period
> >>> of time and never leave it. With batch page replacement there is a
> chance
> >>> to prevent random-LRU page replacement or postpone it.
> >> I understand case you described, but who will pull the switch ? Human,
> >> artificial intelligence ?
> >> You approach assume some triggering from inner, i don`t like this.
> >>
> >>>
> >>>> But request once more, do you really observe such problems with 2.9
> ver
> >> ?
> >>> Any graphs maybe ?
> >>> I don't have production usage feedback after IGNITE-13086, but I doubt
> >>> something changed significantly.
> >>
> >> Lets wait ?:) In any case (Nikolay, Alex) IGNITE-13086 includes yardstik
> >> bench for PR proven, we can use it once more.
> >>
> >> Thanks !
> >>>
> >>>
> >>> чт, 19 нояб. 2020 г. в 09:18, Zhenya Stanilovsky <
> >> arzamas123@mail.ru.invalid
> >>>> :
> >>>
> >>>>
> >>>> Alexey, we already have changes that partially fixes this issue [1]
> >>>> Easy way:
> >>>> Looks like we already have converge in page replacement.
> >>>> If we change 5 times touch iterator from random lru algo into, for
> >>>> example — 7 we will obtain fast improvement from scratch.
> >>>>
> >>>> » Batch page replacement
> >>>> This approach still not applicable for real life if you wan`t to
> observe
> >>>> ugly people for threshold (i.e. 12 h) interval. And, of course, you
> >>>> understand that dramatically reduce of such interval gives nothing?
> >>>>
> >>>> » Change the page replacement algorithm.
> >>>> That`s way i vote for ) But request once more, do you really observe
> >> such
> >>>> problems with 2.9 ver ? Any graphs maybe ?
> >>>>
> >>>> thanks !
> >>>>
> >>>> [1]  https://issues.apache.org/jira/browse/IGNITE-13086
> >>>>> Hello, Igniters!
> >>>>>
> >>>>> Currently, for page replacement (page rotation between page-memory
> and
> >>>>> disk) we use Random-LRU algorithm. It has a low maintenance cost
and
> >>>>> relatively simple implementation, but it has many disadvantages
and
> >>>> affects
> >>>>> performance very much when replacement is started. We even have
> >> warnings
> >>>> in
> >>>>> the log when page replacement started and a special event for this.
I
> >> know
> >>>>> Ignite deployments where administrators force to restart cluster
> nodes
> >>>>> periodically to avoid page replacement.
> >>>>>
> >>>>> I have a couple of proposals to improve page replacement in Ignite:
> >>>>>
> >>>>> *Batch page replacement.*
> >>>>>
> >>>>> Main idea: in some cases start background task to evict cold pages
> from
> >>>>> page-memory (for example, pages, last touched more than 12 hours
> ago).
> >>>>>
> >>>>> The task can be started:
> >>>>> - Automatically, triggered by some events, for example, when we
> expect
> >> a
> >>>>> start of Random-LRU page replacing soon (allocated more than 90%
of
> >>>>> page-memory) + we have enough amount of cold pages (we need some
> >> metric to
> >>>>> calculate the number of cold pages) + some time passed since last
> batch
> >>>>> page replacement (to avoid too much resource consumption by
> background
> >>>>> batch replacement).
> >>>>> - Manually (JMX or control.sh), if an administrator wants to control
> >> the
> >>>>> time of batch replacement more precisely (for example, to avoid
the
> >> start
> >>>>> of this task during peak time).
> >>>>>
> >>>>> Batch page replacement will be helpful in some workloads (when some
> >> data
> >>>>> much colder than another), it can prevent the starting of Random-LRU
> >> page
> >>>>> replacement, or if Random-LRU already started it can provide
> >> conditions to
> >>>>> stop it.
> >>>>>
> >>>>> *Change the page replacement algorithm.*
> >>>>>
> >>>>> Good page replacement algorithm should satisfy the requirements:
> >>>>> - low page-fault rates for typical workload
> >>>>> - low maintenance cost (low resource consumption to maintain
> additional
> >>>>> structures required for page replacement)
> >>>>> - fast searching of next page for replacement
> >>>>> - sequential scans resistance (one sequential scan should not evict
> all
> >>>>> relatively hot pages from page-memory)
> >>>>>
> >>>>> Our Random-LRU has low maintenance cost and sequential scan
> resistant,
> >> but
> >>>>> to find the next page for replacement in the best case we scan 5
> >> pages, in
> >>>>> the worst case we can scan all data region segment. Also, due to
> random
> >>>>> nature, it's not very effective in predicting the right page for
> >>>>> replacement to minimize the page-fault rate. And it's much time
> >> required
> >>>> to
> >>>>> totally evict old cold data.
> >>>>>
> >>>>> Usually, database management systems and operating systems use
> >>>>> modifications of LRU algorithms. These algorithms have higher
> >> maintenance
> >>>>> costs (pages list should be modified on each page access), but often
> >> they
> >>>>> are effective from a "page-fault rate" point of view and have O(1)
> >>>>> complexity for a searching page to replace. Simple LRU is not
> >> sequential
> >>>>> scan resistant, but modifications that utilize page access frequency
> >> are
> >>>>> resistant to sequential scan.
> >>>>>
> >>>>> We can try one of the modifications of LRU as well (for example,
> >>>> "segmented
> >>>>> LRU" seems suitable for Ignite).
> >>>>>
> >>>>> Ignite is a memory-centric product, so "low maintenance cost" is
very
> >>>>> critical. And there is a risk that page replacement algorithm can
> >> affect
> >>>>> workloads, where page replacement is not used (enough RAM to store
> all
> >>>>> data). Of course, any page replacement solution should be carefully
> >>>>> benchmarked.
> >>>>>
> >>>>>
> >>>>> Igniters, WDYT? If any of these proposals look reasonable to you,
I
> >> will
> >>>>> create IEP and start implementation.
> >>>>>
> >>>>> Also, I have a draft implementation of system view to determine
how
> hot
> >>>> are
> >>>>> pages in page-memory [1]. I think it will be useful for any of these
> >>>>> approaches (and even if we decide to left page replacement as is).
> >>>>>
> >>>>> [1]:  https://issues.apache.org/jira/browse/IGNITE-13726
> >>>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>
> >>
> >>
> >>
>
>

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