High Peak Memory During Bulk OCR

1. Cap OCR concurrency

Clamp the number of simultaneous OCR tasks to a small ceiling. Fewer concurrent tasks mean fewer full-page bitmaps in memory at once, which directly lowers the peak. Tune the ceiling to the machine's capability.

// Clamp concurrency to avoid memory saturation and CPU over-subscription.
int concurrency = Math.Clamp(Environment.ProcessorCount / 2, 1, 4);

// Clamp concurrency to avoid memory saturation and CPU over-subscription.
int concurrency = Math.Clamp(Environment.ProcessorCount / 2, 1, 4);

Imports System

' Clamp concurrency to avoid memory saturation and CPU over-subscription.
Dim concurrency As Integer = Math.Clamp(Environment.ProcessorCount \ 2, 1, 4)

2. Pool the engines

Create exactly one IronTesseract engine per concurrent slot at startup and reuse them across every segment, rather than constructing a new engine and reloading the language model each time.

// Pre-create one engine per concurrent slot and reuse them across segments.
var enginePool = new ConcurrentBag<IronTesseract>(
    Enumerable.Range(0, concurrency).Select(_ => new IronTesseract())
);

// Pre-create one engine per concurrent slot and reuse them across segments.
var enginePool = new ConcurrentBag<IronTesseract>(
    Enumerable.Range(0, concurrency).Select(_ => new IronTesseract())
);

' Pre-create one engine per concurrent slot and reuse them across segments.
Dim enginePool As New ConcurrentBag(Of IronTesseract)(
    Enumerable.Range(0, concurrency).Select(Function(_) New IronTesseract())
)

Building the pool once amortizes the language-model load cost across the whole run instead of paying it per segment.

3. Gate work with a semaphore

Initialize a SemaphoreSlim to the concurrency limit and wrap it in using. Each task calls WaitAsync() before it starts and Release() in a finally, so only the allowed number of segments are ever in flight at once.

using var semaphore = new SemaphoreSlim(concurrency);
await semaphore.WaitAsync();
try
{
    // Rent a pre-loaded engine from the pool.
    if (!enginePool.TryTake(out var ocr))
        ocr = new IronTesseract(); // Defensive fallback; should never be reached.
    try
    {
        using var input = new OcrInput();
        input.LoadPdf(segmentStream); // page-range segment produced upstream
        var ocrResult = await ocr.ReadAsync(input);
        ocrResult.SaveAsSearchablePdf(outputPath);
    }
    finally
    {
        enginePool.Add(ocr); // Return engine to pool for the next waiting segment.
    }
}
finally
{
    semaphore.Release();
}

using var semaphore = new SemaphoreSlim(concurrency);
await semaphore.WaitAsync();
try
{
    // Rent a pre-loaded engine from the pool.
    if (!enginePool.TryTake(out var ocr))
        ocr = new IronTesseract(); // Defensive fallback; should never be reached.
    try
    {
        using var input = new OcrInput();
        input.LoadPdf(segmentStream); // page-range segment produced upstream
        var ocrResult = await ocr.ReadAsync(input);
        ocrResult.SaveAsSearchablePdf(outputPath);
    }
    finally
    {
        enginePool.Add(ocr); // Return engine to pool for the next waiting segment.
    }
}
finally
{
    semaphore.Release();
}

Imports System.Threading
Imports IronOcr

Dim semaphore As New SemaphoreSlim(concurrency)
Await semaphore.WaitAsync()
Try
    ' Rent a pre-loaded engine from the pool.
    Dim ocr As IronTesseract = Nothing
    If Not enginePool.TryTake(ocr) Then
        ocr = New IronTesseract() ' Defensive fallback; should never be reached.
    End If
    Try
        Using input As New OcrInput()
            input.LoadPdf(segmentStream) ' page-range segment produced upstream
            Dim ocrResult = Await ocr.ReadAsync(input)
            ocrResult.SaveAsSearchablePdf(outputPath)
        End Using
    Finally
        enginePool.Add(ocr) ' Return engine to pool for the next waiting segment.
    End Try
Finally
    semaphore.Release()
End Try

The WaitAsync() call blocks until a slot frees up, and returning the engine in the inner finally hands a pre-loaded engine straight to the next waiting segment.

4. Dispose OcrInput per segment

Wrap each OcrInput in using so its rendered page bitmaps are released the moment the segment is read, before the next task claims the slot.