Gary Potagal created PDFBOX-4188:
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             Summary:  "Maximum allowed scratch file memory exceeded." 
Exception when merging large number of small PDFs
                 Key: PDFBOX-4188
                 URL: https://issues.apache.org/jira/browse/PDFBOX-4188
             Project: PDFBox
          Issue Type: Improvement
    Affects Versions: 2.0.9
            Reporter: Gary Potagal


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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