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https://issues.apache.org/jira/browse/CASSANDRA-7546?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14072180#comment-14072180
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Benedict edited comment on CASSANDRA-7546 at 7/23/14 7:19 PM:
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Well, actually the scheme I outlined isn't _exactly_ requiring a rate of
100MB/s; all that actually needs to happen is it consistently exceed a rate of
1Mb/s for a total allocation of 100MB (which can happen if > 100MB are
allocated in < 1 second, i.e. 100MB/s, but also if 110Mb is allocated over
10s). We can tweak those numbers however we like (within some window of
representable numbers with enough range). For instance exceed a rate of 10MB/s
consistently by a total of 10MB, which would require e.g. dividing our bytes
allocated by 1k, measuring time in 100ns intervals, and offset the present by
10 * 1024. To capture a rate of 100MB/s, we would need to either expect that
memtables never live for more than 0.5 days (probably reasonable, i.e.
represent time in 10ns intervals) or require that a single mutator allocates
10k in one run (also quite reasonable) but we're pushing the limits of what we
can safely represent.
bq. nanoTime is not monotonic
It is monotonic; that's its main purpose. Although there are no doubt caveats
on a given machine/processor for how strictly that is guaranteed
bq. which clones are you talking about
Mistype. I mean the number/size of objects we estimate we've allocated
wastefully for the collection (snap tree / btree). We can estimate this in 2.0
with 200+100*lg2(N), and in 2.1 we measure it exactly.
was (Author: benedict):
Well, actually the scheme I outlined isn't _exactly_ requiring a rate of
100MB/s; all that actually needs to happen is it consistently exceed a rate of
1Mb/s for a total allocation of 100MB (which can happen if > 100MB are
allocated in < 1 second, i.e. 100MB/s, but also if 110Mb is allocated over
10s). We can tweak those numbers however we like (within some window of
representable numbers with enough range). For instance exceed a rate of 10MB/s
consistently by a total of 10MB, which would require e.g. dividing our bytes
allocated by 1k, measuring time in 100ns intervals, and offset the present by
10 * 1024. To capture a rate of 100MB/s, we would need to either expect that
memtables never live for more than 0.5 days (probably reasonable, i.e.
represent time in 10ns intervals) or require that a single mutator allocates
10k in one run (also quite reasonable) but we're pushing the limits of what we
can safely represent.
bq. nanoTime is not monotonic
It is monotonic; that's its main purpose. Although there are no doubt caveats
on a given machine/processor for how strictly that is guaranteed
bq. which clones are you talking about
Mistype. I mean the number/size of objects we estimate we've allocated
wastefully. We can estimate this in 2.0 with 200+100*lg2(N), and in 2.1 we
measure it exactly.
> AtomicSortedColumns.addAllWithSizeDelta has a spin loop that allocates memory
> -----------------------------------------------------------------------------
>
> Key: CASSANDRA-7546
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7546
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: graham sanderson
> Assignee: graham sanderson
> Attachments: 7546.20.txt, 7546.20_2.txt, 7546.20_3.txt,
> 7546.20_alt.txt, suggestion1.txt, suggestion1_21.txt
>
>
> In order to preserve atomicity, this code attempts to read, clone/update,
> then CAS the state of the partition.
> Under heavy contention for updating a single partition this can cause some
> fairly staggering memory growth (the more cores on your machine the worst it
> gets).
> Whilst many usage patterns don't do highly concurrent updates to the same
> partition, hinting today, does, and in this case wild (order(s) of magnitude
> more than expected) memory allocation rates can be seen (especially when the
> updates being hinted are small updates to different partitions which can
> happen very fast on their own) - see CASSANDRA-7545
> It would be best to eliminate/reduce/limit the spinning memory allocation
> whilst not slowing down the very common un-contended case.
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