Maruan Sahyoun commented on PDFBOX-4182:

[~pasfilip] [~tilman] what about this approach: within PDFMergerUtility we 
develop a new merge method which is targeted to be able to close the PDDocument 
after it has been merged. There will be a flag allowing one to select between 
the new and old merge. One needs to select the old merge to get all the current 
capabilities but over time we add to the new merge method. After doing this we 
will need to look into doing further optimizations, such as a 
different/new/improved 'cache'/ScratchFile to reduce the memory consumption 
further if it might still be needed. This way we will have the ability to 
select the current implementation to handle the special cases for which 
(currently) the PDDocument needs to be available but also have a 'slim' method 
if one only wants to merge basic documents. WDYT?

[~pasfilip] I understand that you can't share the documents. Would it be 
possible to provide a sample set done from scratch which reflects the document 
set you are having. Pointing to publicly available documents is also fine. 
Please keep the elements needed to the bare minimum. 

> Improve memory usage of PDFMergerUtility
> ----------------------------------------
>                 Key: PDFBOX-4182
>                 URL: https://issues.apache.org/jira/browse/PDFBOX-4182
>             Project: PDFBox
>          Issue Type: Improvement
>    Affects Versions: 2.0.9
>            Reporter: Pas Filip
>            Priority: Major
>         Attachments: PDFMergerUtilityUsingSupplier.java, Supplier.java, 
> Suppliers.java, 
> failed-merge-utility-4gb-heap-out-of-memory-after-1800-pdfs.png, 
> merge-pdf-stats.xlsx, oom-2gb-heap-after-refactoring-leak-suspect-1.png, 
> oom-2gb-heap-after-refactoring-leak-suspect-2.png, successful - 
> refactored-merge-utility-4gb-heap-2618-files-merged.png, successful 
> -merge-utility-6gb-heap-2618-files-merged.png, 
> successful-merge-utility-6gb-heap-2618-files-merged-setupTempFileOnly.png, 
> successful-merge-utility-8gb-heap-2618-files-merged.png, 
> successful-refactored-merge-utility-4gb-heap-2618-files-merged-setupTempFileOnly.png
> I have been running some tests trying to merge large amounts (2618) of small 
> pdf documents, between 100kb and 130kb, into a single large pdf (288.433kb)
> Memory consumption seems to be the main limitation.
> ScratchFileBuffer seems to consume the majority of the memory usage.
> (see screenshot from mat in attachment)
> (I would include the hprof in attachment so you can analyze yourselves but 
> it's rather large)
> Note that it seems impossible to generate a large pdf using a small memory 
> footprint.
> I personally thought that using MemorySettings with temporary file only would 
> allow me to generate arbitrarily large pdf files but it doesn't seem to help.
> I've run the mergeDocuments with  memory settings:
>  * MemoryUsageSetting.setupMixed(1024L * 1024L, 1024L * 1024L * 1024L * 1024L 
> * 1024L)
>  * MemoryUsageSetting.setupTempFileOnly()
> Refactored version completes with *4GB* heap:
> with temp file only completes 2618 documents in 1.760 min
> *VS*
> *8GB* heap:
> with temp file only completes 2618 documents in 2.0 min
> Heaps of 6gb or less result in OOM. (Didn't try different sizes between 6GB 
> and 8GB)
>  It looks like the loop in the mergeDocuments accumulates PDDocument objects 
> in a list which are closed after the merge is completed.
> Refactoring the code to close these as they are used, instead of accumulating 
> them and closing all at the end, improves memory usage considerably.(although 
> doesn't seem to be eliminated completed based on mat analysis.)
> Another change I've implemented is to only create the inputstream when the 
> file needs to be read and to close it alongside the PDDocument.
> (Some inputstreams contain buffers and depending on the size of the buffers 
> and or the stream type accumulating all the streams is a potential 
> memory-hog.)
> These changes seems to have a beneficial improvement in the sense that I can 
> process the same amount of pdfs with about half the memory.
>  I'd appreciate it if you could roll these changes into the main codebase.
> (I've respected java 6 compatibility.)
> I've included in attachment the java files of the new implementation:
>  * Suppliers
>  * Supplier
>  * PDFMergerUtilityUsingSupplier
> PDFMergerUtilityUsingSupplier can replace the previous version. No signature 
> changes only internal code changes. (just rename the class to 
> PDFMergerUtility if you decide to implemented the changes.)
>  In attachment you can also find some screenshots from visualvm showing the 
> memory usage of the original version and the refactored version as well as 
> some info produced by mat after analysing the heap.
> If you know of any other means, without running into memory issues, to merge 
> large sets of pdf files into a large single pdf I'd love to hear about it!
> I'd also suggest that there should be further improvements made in memory 
> usage in general as pdfbox seems to consumer a lot of memory in general.

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