Function CCH_prefetch

src/jrd/cch.cpp:1849–1904

#ifdef CACHE_READER
void CCH_prefetch(thread_db* tdbb, SLONG* pages, SSHORT count)
{
/**************************************
 *
 *	C C H _ p r e f e t c h
 *
 **************************************
 *
 * Functional description
 *	Given a vector of pages, set corresponding bits
 *	in global prefetch bitmap. Initiate an asynchronous
 *	I/O and get the cache reader reading in our behalf
 *	as well.
 *
 **************************************/
	SET_TDBB(tdbb);
	Database* dbb = tdbb->getDatabase();
	BufferControl* bcb = dbb->dbb_bcb;

	if (!count || !(bcb->bcb_flags & BCB_cache_reader))
	{
		// Caller isn't really serious.
		return;
	}

	// Switch default pool to permanent pool for setting bits in prefetch bitmap.
	Jrd::ContextPoolHolder context(tdbb, bcb->bcb_bufferpool);

	// The global prefetch bitmap is the key to the I/O coalescense mechanism which dovetails
	// all thread prefetch requests to minimize sequential I/O requests.
	// It also enables multipage I/O by implicitly sorting page vector requests.

	SLONG first_page = 0;
	for (const SLONG* const end = pages + count; pages < end;)
	{
		const SLONG page = *pages++;
		if (page)
		{
			SBM_set(tdbb, &bcb->bcb_prefetch, page);
			if (!first_page)
				first_page = page;
		}
	}

	// Not likely that the caller's page vector was empty but check anyway.

	if (first_page)
	{
		prf prefetch;

		prefetch_init(&prefetch, tdbb);
		prefetch_prologue(&prefetch, &first_page);
		prefetch_io(&prefetch, tdbb->tdbb_status_vector);
		prefetch_epilogue(&prefetch, tdbb->tdbb_status_vector);
	}
}