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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