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From 谢良 <xieli...@xiaomi.com>
Subject 答复: Does HBase RegionServer benefit from OS Page Cache
Date Tue, 26 Mar 2013 03:01:23 GMT
Maybe we should adopt some ideas from RDBMS ?
In MySQL area:
Innodb storage engine has a buffer pool(just like current block cache), caches both
compressed and uncompressed pages in latest innodb version, it brings
about adaptive LRU algorithm, see http://dev.mysql.com/doc/innodb/1.1/en/innodb-compression-internals.html,
in short, it's somehow more subtle for this detail than leveldb&hbase's implementation,

per my view. In deed, we(Xiaomi) had a plan to develop&evaluate it already
(we logged it in our internal phabricator system before), hopefully we could contribute it

to community in the future.

Another storage engine Falcon has "Row Cache" feature, which similar with Enis mentioned,

It's more friendly against random read scenario.
Every user table could choose a prefered storage engine in MySQL, so here, my point is:
maybe we need to consider supporting more configureable cache strategy per table granularity

Regards,
Liang
________________________________________
发件人: Enis Söztutar [enis.soz@gmail.com]
发送时间: 2013年3月26日 4:26
收件人: hbase-user
Cc: lars hofhansl
主题: Re: Does HBase RegionServer benefit from OS Page Cache

> With very large heaps and a GC that can handle them (perhaps the G1 GC),
another option which might be worth experimenting with is a value (KV)
cache independent of the block cache which could be enabled on a per-table
basis
Thanks Andy for bringing this up. We've had some discussions some time ago
about a row-cache (or KV cache)
http://search-hadoop.com/m/XTlxT1xRtYw/hbase+key+value+cache+from%253Aenis&subj=RE+keyvalue+cache

The takeaway was that if you are mostly doing point gets, rather than
scans, this cache might be better.

> 1) [HBASE-7404]: L1/L2 block cache
I knew about the Bucket cache, but not that bucket cache could hold
compressed blocks. Is it the case, or are you suggesting we can add that to
this L2 cache.

>  2) [HBASE-5263] Preserving cached data on compactions through
cache-on-write
Thanks, this is the same idea. I'll track the ticket.

Enis


On Mon, Mar 25, 2013 at 12:18 PM, Liyin Tang <liyin.tang@fb.com> wrote:

> Hi Enis,
> Good ideas ! And hbase community is driving on these 2 items.
> 1) [HBASE-7404]: L1/L2 block cache
> 2) [HBASE-5263] Preserving cached data on compactions through
> cache-on-write
>
> Thanks a lot
> Liyin
> ________________________________________
> From: Enis Söztutar [enis.soz@gmail.com]
> Sent: Monday, March 25, 2013 11:24 AM
> To: hbase-user
> Cc: lars hofhansl
> Subject: Re: Does HBase RegionServer benefit from OS Page Cache
>
> Thanks Liyin for sharing your use cases.
>
> Related to those, I was thinking of two improvements:
>  - AFAIK, MySQL keeps the compressed and uncompressed versions of the blocs
> in its block cache, failing over the compressed one if decompressed one
> gets evicted. With very large heaps, maybe keeping around the compressed
> blocks in a secondary cache makes sense?
>  - A compaction will trash the cache. But maybe we can track keyvalues
> (inside cached blocks are cached) for the files in the compaction, and mark
> the blocks of the resulting compacted file which contain previously cached
> keyvalues to be cached after the compaction. I have to research the
> feasibility of this approach.
>
> Enis
>
>
> On Sun, Mar 24, 2013 at 10:15 PM, Liyin Tang <liyin.tang@fb.com> wrote:
>
> > Block cache is for uncompressed data while OS page contains the
> compressed
> > data. Unless the request pattern is full-table sequential scan, the block
> > cache is still quite useful. I think the size of the block cache should
> be
> > the amont of hot data we want to retain within a compaction cycle, which
> is
> > quite hard to estimate in some use cases.
> >
> >
> > Thanks a lot
> > Liyin
> > ________________________________________
> > From: lars hofhansl [larsh@apache.org]
> > Sent: Saturday, March 23, 2013 10:20 PM
> > To: user@hbase.apache.org
> > Subject: Re: Does HBase RegionServer benefit from OS Page Cache
> >
> > Interesting.
> >
> > > 2) The blocks in the block cache will be naturally invalid quickly
> after
> > the compactions.
> >
> > Should one keep the block cache small in order to increase the OS page
> > cache?
> >
> > Does you data suggest we should not use the block cache at all?
> >
> >
> > Thanks.
> >
> > -- Lars
> >
> >
> >
> > ________________________________
> >  From: Liyin Tang <liyintang2011@gmail.com>
> > To: user@hbase.apache.org
> > Sent: Saturday, March 23, 2013 9:44 PM
> > Subject: Re: Does HBase RegionServer benefit from OS Page Cache
> >
> > We (Facebook) are closely monitoring the OS page cache hit ratio in the
> > production environments. My experience is if your data access pattern is
> > very random, then the OS page cache won't help you so much even though
> the
> > data locality is very high. On the other hand, if the requests are always
> > against the recent data points, then the page cache hit ratio could be
> much
> > higher.
> >
> > Actually, there are lots of optimizations could be done in HDFS. For
> > example, we are working on fadvice away the 2nd/3rd replicated data from
> OS
> > page cache so that it potentially could improve your OS page cache by 3X.
> > Also, by taking advantage of the tiered-based compaction+fadvice in HDFS,
> > the region server could keep more hot data in OS page cache based on the
> > read access pattern.
> >
> > Another separate point is that we probably should NOT reply on the
> > memstore/block cache to keep hot data. 1) The more data in the memstore,
> > the more data the region server need to recovery from the server
> failures.
> > So the tradeoff is the recovery time. 2) The blocks in the block cache
> will
> > be naturally invalid quickly after the compactions. So region server
> > probably won't be benefit from large JVM size at all.
> >
> > Thanks a lot
> > Liyin
> >
> > On Sat, Mar 23, 2013 at 6:13 PM, Ted Yu <yuzhihong@gmail.com> wrote:
> >
> > > Coming up is the following enhancement which would make MSLAB even
> > better:
> > >
> > > HBASE-8163 MemStoreChunkPool: An improvement for JAVA GC when using
> MSLAB
> > >
> > > FYI
> > >
> > > On Sat, Mar 23, 2013 at 5:31 PM, Pankaj Gupta <pankaj.roark@gmail.com
> > > >wrote:
> > >
> > > > Thanks a lot for the explanation. It's good to know that MSlab is
> > stable
> > > > and safe to enable (we don't have it enable right now, we're using
> > 0.92).
> > > > This would allow us to more freely allocate memory to HBase. I really
> > > > enjoyed the depth of explanation from both Enis and J-D. I was indeed
> > > > mistakenly referring to HFile as HLog, fortunately you were still
> able
> > > > understand my question.
> > > >
> > > > Thanks,
> > > > Pankaj
> > > > On Mar 21, 2013, at 1:28 PM, Enis Söztutar <enis.soz@gmail.com>
> wrote:
> > > >
> > > > > I think the page cache is not totally useless, but as long as you
> can
> > > > > control the GC, you should prefer the block cache. Some of the
> > reasons
> > > of
> > > > > the top of my head:
> > > > > - In case of a cache hit, for OS cache, you have to go through the
> DN
> > > > > layer (an RPC if ssr disabled), and do a kernel jump, and read
> using
> > > the
> > > > > read() libc vs  for reading a block from the block cache, only the
> > > HBase
> > > > > process is involved. There is no process switch involved and no
> > kernel
> > > > > jumps.
> > > > > - The read access path is optimized per hfile block. FS page
> > boundaries
> > > > > and hfile block boundaries are not aligned at all.
> > > > > - There is very little control to the page cache to cache / not
> cache
> > > > > based on expected access patterns. For example, we can mark META
> > region
> > > > > blocks, and some column families, and hfile index blocks always
> > cached
> > > or
> > > > > cached with high priority. Also, for full table scans, we can
> > > explicitly
> > > > > disable block caching to not trash the current working set. With
OS
> > > page
> > > > > cache, you do not have this control.
> > > > >
> > > > > Enis
> > > > >
> > > > >
> > > > > On Wed, Mar 20, 2013 at 10:30 AM, Jean-Daniel Cryans <
> > > > jdcryans@apache.org>wrote:
> > > > >
> > > > >> First, MSLAB has been enabled by default since 0.92.0 as it was
> > deemed
> > > > >> stable enough. So, unless you are on 0.90, you are already using
> it.
> > > > >>
> > > > >> Also, I'm not sure why you are referencing the HLog in your first
> > > > >> paragraph in the context of reading from disk, because the HLogs
> are
> > > > >> rarely read (only on recovery). Maybe you meant HFile?
> > > > >>
> > > > >> In any case, your email covers most arguments except for one:
> > > > >> checksumming. Retrieving a block from HDFS, even when using short
> > > > >> circuit reads to go directly to the OS instead of passing through
> > the
> > > > >> DN, will take quite a bit more time than reading directly from
the
> > > > >> block cache. This is why even if you disable block caching on
a
> > family
> > > > >> that the index and root blocks will still be block cached, as
> > reading
> > > > >> those very hot blocks from disk would take way too long.
> > > > >>
> > > > >> Regarding your main question (how does the OS buffer help?),
I
> don't
> > > > >> have a good answer. It kind of depends on the amount of RAM you
> have
> > > > >> and what your workload is like. As a data point, I've been
> > successful
> > > > >> running with 24GB of heap (50% dedicated to the block cache)
with
> a
> > > > >> workload consisting mainly of small writes, short scans, and
a
> > typical
> > > > >> random read distribution for a website. I can't remember the
last
> > time
> > > > >> I saw a full GC and it's been running for more than a year like
> > this.
> > > > >>
> > > > >> Hope this somehow helps,
> > > > >>
> > > > >> J-D
> > > > >>
> > > > >> On Wed, Mar 20, 2013 at 12:34 AM, Pankaj Gupta <
> > > pankaj.roark@gmail.com>
> > > > >> wrote:
> > > > >>> Given that HBase has it's own cache (block cache and bloom
> filters)
> > > and
> > > > >> that all the table data is stored in HDFS, I'm wondering if HBase
> > > > benefits
> > > > >> from OS page cache at all. In the set up I'm using HBase Region
> > > Servers
> > > > run
> > > > >> on the same boxes as the HDFS data node. In such a scenario if
the
> > > > >> underlying HLog files lives on the same machine then having a
> > healthy
> > > > >> memory surplus may mean that the data node can serve underlying
> file
> > > > from
> > > > >> page cache and thus improving HBase performance. Is this really
> the
> > > > case?
> > > > >> (I guess page cache should also help in case where HLog file
lives
> > on
> > > a
> > > > >> different machine but in that case network I/O will probably
drown
> > the
> > > > >> speedup achieved due to not hitting the disk.
> > > > >>>
> > > > >>> I'm asking because if page cache were useful then in an HBase
set
> > up
> > > > not
> > > > >> utilizing all the memory on the machine for the region server
may
> > not
> > > be
> > > > >> that bad. The reason one would not want to use all the memory
for
> > > region
> > > > >> server would be long garbage collection pauses that large heap
> size
> > > may
> > > > >> induce. I understand that work has been done to fix the long
> pauses
> > > > caused
> > > > >> due to memory fragmentation in the old generation, mostly
> concurrent
> > > > >> garbage collector by using slab cache allocator for memstore
but
> > that
> > > > >> feature is marked experimental and we're not ready to take risks
> > yet.
> > > > So if
> > > > >> the page cache was useful in any way on Region Servers we could
go
> > > with
> > > > >> less memory for RegionServer process with the understanding that
> > free
> > > > >> memory on the machine is not completely going to waste. Thus
my
> > > > curiosity
> > > > >> about utility of os page cache to performance of HBase.
> > > > >>>
> > > > >>> Thanks in Advance,
> > > > >>> Pankaj
> > > > >>
> > > >
> > > >
> > >
> >
>
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