key + 1b
val + 2b
item struct 48b
[optional CAS] 8b
then the suffix trailer printf: please read the manpage for snprintf:
snprintf(suffix, 40, " %d %d\r\n", flags, nbytes - 2);
there are four characters in there. two spaces, and then \r and \n. the
two %d's change into: "0" for flags, and "90" for bytes (ignore the -2 in
there). that's 7b total.
In your no-cas test: 5 + 92 + 48 + 7 == 152. that's the exact size of slab
3. (152b).
On Mon, 16 Nov 2015, Nicolas Martinez wrote:
> The test was :
> •Key : test
> •Data : 90 bytes
> •Flags : 0
>
> :(
>
> Le lundi 16 novembre 2015 19:01:50 UTC+1, Nicolas Martinez a écrit :
> Ok... i think i have understood what you say:
>
> Key (Characters Number) + 1 + Data (Characters Number) + Header + Chunk
> Size + CAS Size
>
> Header = Flags (Characters Number) + Key (Characters Numbers)
>
> + 2 bytes ( \r\n )
>
> + 4 bytes (2 spaces and 1 \r)
>
> Chunk Size = 48 bytes (default)
>
> CAS Size = 8 bytes (64 bits platform)
>
>
> Seems to be good:
>
> 4 + 1 + 90 + (1+4+2+4) + 48 + 8 = 162 Bytes => Slab 4 (192 Bytes)
>
>
> But, if i start Memcached with -C, it's wrong.
>
> 4 + 1 + 90 + (1+4+2+4) + 48 = 154 Bytes => Slab 3 (152 Bytes)
>
>
> It must be in Slab4 (192 Bytes) No?
>
>
> Le lundi 16 novembre 2015 15:37:59 UTC+1, Nicolas Martinez a écrit :
> Thank you very much for yours answers.Ok for CAS... i don't use
> -C so i have to add 8 bytes
>
> i still don"t understand these lines :
> >> 2b appended to data_bytes for an extra "\r\n" +
> So, \r\n == 2b ?
>
> >> + 4 bytes for spaces and \r\n
> which spaces?
> What is this \r\n ? Isn't already counted before?
> There is 1 "\r\n" and 1 "\"
>
> echo -e 'set 30 0 3600 30\r\n'$data'\r'| nc ${memcached_server} 11211
>
>
> With your example:
> * Key : 2c
> * Val : 28 bytes
> * Flg : 0 (1bytes)
>
> turns into:
> * Key : 3b
>
> => key number characters + 1
> * Val : 30b
>
> => 28 bytes + 2 bytes ("\r\n")
> * Hdr : 4b + 3b == 7b
>
> => What are 4b?
> => 3b are: flags (1b) + ??
> * Itm : 56b
>
> => 48b + 8b (CAS)
> => 96b. which is the cap for slab 1 in a default setup.
>
>
>
> Thank you again.
>
> Le lundi 16 novembre 2015 00:31:07 UTC+1, Dormando a écrit :
> Read carefully:
>
> item_size_ok(const size_t nkey, const int flags, const int nbytes) {
>
> passes:
>
> size_t ntotal = item_make_header(nkey + 1, flags, nbytes,
> prefix, &nsuffix);
>
> Then conditionally:
>
> if (settings.use_cas) {
> ntotal += sizeof(uint64_t);
> }
>
> item_make_header is doing:
> nsuffix = (uint8_t) snprintf(suffix, 40, " %d %d\r\n", flags, nbytes -
> 2);
>
> Then:
>
> return sizeof(item) + nkey + *nsuffix + nbytes;
>
> It's convoluted but shirt.
>
> the lengths are:
> key +
> 1 +
> data_bytes +
> 2b appended to data_bytes for an extra "\r\n" +
> stringified rep of the flags + data length
> + 4 bytes for spaces and \r\n (these are carriage returns, one byte
> each)
> + 8b for CAS if enabled
>
> CAS can be turned off via the -C starttime arg. it takes up 8 bytes.
>
> Example:
> * Key : 2c
> * Val : 28b
> * Flg : 0 (1b)
>
> turns into:
> * Key : 3b
> * Val : 30b
> * Hdr : 4b + 3b == 7b
> * Itm : 56b
> => 96b. which is the cap for slab 1 in a default setup.
>
> It's tough to get it exact for small chunks due to the way the header is
> added. You should ballpark or tune the -f value to align with your
> observed data.
>
> On Sun, 15 Nov 2015, Nicolas Martinez wrote:
>
> > Hi,
> > Is CAS always used?
> > If yes, we have to always add 56 bytes to the KEY and VALUE ?
> > you don't count FLAGS characters??
> >
> > I've found that Flags's size (number of characters) impact the
> storage.
> >
> > Example:
> > * Key : 2 characters = 2 bytes
> > * Value : 28 characters = 28 bytes
> > * FLAGS : 1 characters = 1 bytes
> > => 31 bytes
> >
> > seems to take the same storage as
> > * Key : 1 characters = 1 bytes
> > * Value : 28 characters = 28 bytes
> > * FLAGS : 2 characters = 2 bytes
> > => 31 bytes ... wich is the limit to be stored in Slab1
> >
> > ok for the /r/n ... should take 4 bytes no?
> >
> > So, if we count 56 bytes for CAS :
> 56(cas)+31(key+value+flags)+4(/r/n)= 91
> >
> > Not good... :(
> >
> > where I'm wrong ??
> >
> > Le samedi 14 novembre 2015 23:55:12 UTC+1, Dormando a écrit :
> > The mysql docs don't speak for the main tree... that's their
> own thing.
> >
> > the "sizes" binary that comes with the source tree tells you
> how many
> > bytes an item will use (though I intend to add this output to
> the 'stats'
> > output somewhere). With CAS this is 56 bytes.
> >
> > 56 + 2 + 30 == 88. Class 1 by default (in 1.4.24) is 96 bytes,
> but the
> > item still ends up in class 2.
> >
> > Why is this? (unfortunately?) because memcached pre-renders
> part of the
> > text protocol into the item header:
> >
> > *nsuffix = (uint8_t) snprintf(suffix, 40, " %d %d\r\n", flags,
> nbytes -
> > 2);
> > return sizeof(item) + nkey + *nsuffix + nbytes;
> >
> > so the flags + length are getting flattened + \r\n added to the
> end.
> > Together that's just enough to push it over the edge. It'd also
> be nice to
> > add a highly optimized numerics printf so I could twiddle
> options to save
> > a few bytes of memory in objects, but don't get your hopes up
> for that
> > happening soon :)
> >
> > On Sat, 14 Nov 2015, Nicolas Martinez wrote:
> >
> > > Add: Memcached version : 1.4.4 (RedHat)
> > >
> > > Le samedi 14 novembre 2015 14:49:37 UTC+1, Nicolas Martinez a
> écrit :
> > > Hi, few days i'm reading Memcached documentation and
> blogs... and i don't understand how objects are stored.
> > >
> > > My test
> > >
> > > 3 slabs :
> > >
> > > * 96.0 Bytes
> > > * 120.0 Bytes
> > > * 240.0 Bytes
> > > Everywhere, it's told :
> > > * if data is < 96 Bytes, it will be stored in Slabs1 (96B)
> > > * if data > 96B and < 120B, it will be stored in Slabs2
> (120B)
> > > * if data > 120B, it will be stored in Slabs3 (240B)
> > > * etc.
> > > BUT, for example, when i'm creating an 30B object, it's
> stored in Slab2 (120B), and NOT in Slab1 (96B).
> > >
> > > External sources:
> > > For example, the default size for the smallest block is
> 88 bytes (40 bytes of value, and the default 48 bytes for the key and flag
> data). If the size of the first item you store into the cache is less than 40
> bytes, then a slab with a block size of 88 bytes is created and the value
> stored.
> > > =>
> https://dev.mysql.com/doc/mysql-ha-scalability/en/ha-memcached-using-memory.html
> > >
> > >
> > > WRONG
> > >
> > > A slab class is a collection of pages divided into same
> sized chunks. Each slab class is referenced to by its chunk size. So we’ll
> have Slab class 80kb, Slab class 100kb and so on. When an object needs to be
> stored, its size determines where it gets stored. So if the object is larger
> than 80kb but less than 100kb, it
> gets stored into
> > Slab
> > > class 100kb.
> > > =>
> http://returnfoo.com/2012/02/memcached-memory-allocation-and-optimization-2/
> > >
> > >
> > > WRONG
> > >
> > > How i create an object:
> > >
> > > data=$(pwgen 30 -c 1)
> > > echo -e 'set 30 0 3600 30\r\n'$data'\r'| nc
> ${memcached_server} 11211
> > >
> > >
> > > So, when 30B object is creating :
> > > * key name : 30 = 2 bytes
> > > * value: 30 characters = 30 bytes
> > > * tags : 0 = 1 bytes
> > > => All = 33 bytes
> > > If i add the default 48b as explained on Mysql website : 33 +
> 48 = 81B ... so < Slab1 (91B)... but always stored in Slab2 (120B)
> > >
> > > So, the size used to store object in the good Slab is not:
> > > * object value size
> > > * sum of KEY, VALUE and TAGS in bytes
> > > KEY size : 1 character = 1 B
> > > VALUE size : 1 character = 1 B
> > > TAGS size : 1 character = 1 B
> > >
> > > ... as read everywhere
> > >
> > > So, It seems that: (SUM of KEY+VALUE+TAGS )
> > > * For slab1 96.0 Bytes, data stored if <= 31 B (SUM of
> 2+28+1 )
> > > * For slab2 120.0 Bytes, data stored if <= 55 B (SUM of
> 2+52+1 )
> > > * For slab3 152.0 Bytes, data stored if <= 87 B (SUM of
> 2+84+1 )
> > > * For slab4 192.0 Bytes, data stored if <= 126 B (SUM of
> 3+122+1 )
> > > * For slab5 240.0 Bytes, data stored if <= 174 B (SUM of
> 3+170+1 )
> > > * etc.
> > >
> > > My configuration :
> > > * Chunk Size : 48
> > > * Chunk Growth Factore: 1,25
> > > * Max Bytes: 64.0 MBytes
> > >
> > > So, someone could explain me how the data is stored in the
> right Slabs???
> > > How calculate it???
> > >
> > > Thank you
> > >
> > >
> > > --
> > >
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