Databases
StackOverflow2013
Postsdbo
POLock free multiple readers single writer
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POLock free multiple readers single writer
    primarykey
    data
    text
    <p>I have got an in memory data structure that is read by multiple threads and written by only one thread. Currently I am using a critical section to make this access threadsafe. Unfortunately this has the effect of blocking readers even though only another reader is accessing it.</p> <p>There are two options to remedy this:</p> <ol> <li>use TMultiReadExclusiveWriteSynchronizer</li> <li>do away with any blocking by using a lock free approach</li> </ol> <p>For 2. I have got the following so far (any code that doesn't matter has been left out):</p> <pre><code>type TDataManager = class private FAccessCount: integer; FData: TDataClass; public procedure Read(out _Some: integer; out _Data: double); procedure Write(_Some: integer; _Data: double); end; procedure TDataManager.Read(out _Some: integer; out _Data: double); var Data: TDAtaClass; begin InterlockedIncrement(FAccessCount); try // make sure we get both values from the same TDataClass instance Data := FData; // read the actual data _Some := Data.Some; _Data := Data.Data; finally InterlockedDecrement(FAccessCount); end; end; procedure TDataManager.Write(_Some: integer; _Data: double); var NewData: TDataClass; OldData: TDataClass; ReaderCount: integer; begin NewData := TDataClass.Create(_Some, _Data); InterlockedIncrement(FAccessCount); OldData := TDataClass(InterlockedExchange(integer(FData), integer(NewData)); // now FData points to the new instance but there might still be // readers that got the old one before we exchanged it. ReaderCount := InterlockedDecrement(FAccessCount); if ReaderCount = 0 then // no active readers, so we can safely free the old instance FreeAndNil(OldData) else begin /// here is the problem end; end; </code></pre> <p>Unfortunately there is the small problem of getting rid of the OldData instance after it has been replaced. If no other thread is currently within the Read method (ReaderCount=0), it can safely be disposed and that's it. But what can I do if that's not the case? I could just store it until the next call and dispose it there, but Windows scheduling could in theory let a reader thread sleep while it is within the Read method and still has got a reference to OldData.</p> <p>If you see any other problem with the above code, please tell me about it. This is to be run on computers with multiple cores and the above methods are to be called very frequently.</p> <p>In case this matters: I am using Delphi 2007 with the builtin memory manager. I am aware that the memory manager probably enforces some lock anyway when creating a new class but I want to ignore that for the moment.</p> <p>Edit: It may not have been clear from the above: For the full lifetime of the TDataManager object there is only one thread that writes to the data, not several that might compete for write access. So this is a special case of MREW.</p>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USAdisak
    1. USdummzeuch
    plurals
    1. This table or related slice is empty.
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POLock free multiple readers single writer
      1. USavar
      1. VTFavorite
    2. VO
      singulars
      1. POLock free multiple readers single writer
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POLock free multiple readers single writer
      1. This table or related slice is empty.
      1. VTUpMod
    1. COI'm wary of self-written lock-free code, it's nearly impossible to get it right. As for TMREWS: There's no way around timing your use-case on typical machines, as there are different ways to implement them, and the VCL gives you only one. For an article comparing different implementations (including timing) see http://www.codeproject.com/KB/threads/testing_rwlocks.aspx
      singulars
      1. POLock free multiple readers single writer
      1. USmghie
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload