Cool, thanks for the update. On Friday, July 23, 2010, David T. Lewis <lewis@mail.msen.com> wrote:
Yes, it really is true. That said, I would expect the practical impact to vary depending on how busy the main image is.
If you are running a Squeak/Pharo image that is not busy, such as your own personal desktop image or a Seaside image that is not currently receiving requests, then the additional system memory used by the forked background image is extremely small (it's pretty amazing actually, try watching it on a system monitor such as xosview). However, I would expect that if you were to do this with a busy Seaside image, then that image would be doing a lot of updates to memory and possibly garbage collection, so that will force the OS to use more memory.
Unfortunately this means that the busier your image is, the less benefit you get from a memory usage point of view. But even at that, you still get the advantage of doing the image save in a background OS process running at lower OS scheduling priority.
I do not actually run any Seaside server images, so someone would need to try this on a real system to find out how much benefit it delivers in practice. I would be interested to know the results.
Dave
On Fri, Jul 23, 2010 at 02:12:14PM +0200, Lukas Renggli wrote:
David, is what is said about memory management really true? I thought that saving the image triggered a garbage collect what forces to copy the whole memory anyway.
Lukas
On 23 July 2010 14:00, David T. Lewis <lewis@mail.msen.com> wrote:
On Fri, Jul 23, 2010 at 09:49:46AM +0200, Lukas Renggli wrote:
But How I can do that without fork? I track changes in another process and when I do update I save image in this process. How I can initiate image saving from ui process?
I don't understand what you are saying.
The thing is that using #fork to save the image has no effect (other than wrecking the image), because #snapshot:andQuit: is eventually calling a primitive that locks the complete VM anyway.
If you want to fork while saving the image you need to use an OS-level fork, not a Smalltalk-level fork. You can do this with OSProcess for example (check the Seaside mailing-list archive).
"UnixProcess saveImageInBackgroundNicely"
Explained in the method comment:
UnixProcess class>>saveImageInBackground: savedImageName nice: niceFlag ? ? ? ?"When Squeak is used as a server it is sometimes desirable to periodically ? ? ? ?save image snapshots. This method forks a headless Squeak to perform a ? ? ? ?snapshot without impacting the server Squeak. Very little additional memory ? ? ? ?is required to do this because Unix copy-on-write memory management allows ? ? ? ?the two Squeak images to share object memory while the save is performed. ? ? ? ?The saved image is given a time stamped name, and the image name of ? ? ? ?the main server Squeak remains unchanged. If niceFlag is true, the ? ? ? ?background OS process runs at lowered scheduling priority."
Dave
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