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From Vladimir Ozerov <voze...@gridgain.com>
Subject Re: Cache scan efficiency
Date Sun, 16 Sep 2018 16:54:06 GMT
HI Alex,

This is good that you observed speedup. But I do not think this solution
works for the product in general case. Amount of RAM is limited, and even a
single partition may need more space than RAM available. Moving a lot of
pages to page memory for scan means that you evict a lot of other pages,
what will ultimately lead to bad performance of subsequent queries and
defeat LRU algorithms, which are of great improtance for good database
performance.

Database vendors choose another approach - skip BTrees, iterate direclty
over data pages, read them in multi-block fashion, use separate scan buffer
to avoid excessive evictions of other hot pages. Corresponding ticket for
SQL exists [1], but idea is common for all parts of the system, requiring
scans.

As far as proposed solution, it might be good idea to add special API to
"warmup" partition with clear explanation of pros (fast scan after warmup)
and cons (slowdown of any other operations). But I think we should not make
this approach part of normal scans.

Vladimir.

[1] https://issues.apache.org/jira/browse/IGNITE-6057


On Sun, Sep 16, 2018 at 6:44 PM Alexei Scherbakov <
alexey.scherbakoff@gmail.com> wrote:

> Igniters,
>
> My use case involves scenario where it's necessary to iterate over
> large(many TBs) persistent cache doing some calculation on read data.
>
> The basic solution is to iterate cache using ScanQuery.
>
> This turns out to be slow because iteration over cache involves a lot of
> random disk access for reading data pages referenced from leaf pages by
> links.
>
> This is especially true when data is stored on disks with slow random
> access, like SAS disks. In my case on modern SAS disks array reading speed
> was like several MB/sec while sequential read speed in perf test was about
> GB/sec.
>
> I was able to fix the issue by using ScanQuery with explicit partition set
> and running simple warmup code before each partition scan.
>
> The code pins cold pages in memory in sequential order thus eliminating
> random disk access. Speedup was like x100 magnitude.
>
> I suggest adding the improvement to the product's core  by always
> sequentially preloading pages for all internal partition iterations (cache
> iterators, scan queries, sql queries with scan plan) if partition is cold
> (low number of pinned pages).
>
> This also should speed up rebalancing from cold partitions.
>
> Ignite JIRA ticket [1]
>
> Thoughts ?
>
> [1] https://issues.apache.org/jira/browse/IGNITE-8873
>
> --
>
> Best regards,
> Alexei Scherbakov
>

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