Mark TX_COMMIT transaction with TXG_NOTHROTTLE.

TX_COMMIT has no on-disk representation and does not produce any more
dirty data.  It should not wait for anything, and even just skipping
the checks if not waiting gives improvement noticeable in profiler.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Prakash Surya <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #14798

Author: amotin

module/zfs/zil.c

@@ -3155,7 +3155,14 @@ static void
 zil_commit_itx_assign(zilog_t *zilog, zil_commit_waiter_t *zcw)
 {
 	dmu_tx_t *tx = dmu_tx_create(zilog->zl_os);
-	VERIFY0(dmu_tx_assign(tx, TXG_WAIT));
+
+	/*
+	 * Since we are not going to create any new dirty data, and we
+	 * can even help with clearing the existing dirty data, we
+	 * should not be subject to the dirty data based delays. We
+	 * use TXG_NOTHROTTLE to bypass the delay mechanism.
+	 */
+	VERIFY0(dmu_tx_assign(tx, TXG_WAIT | TXG_NOTHROTTLE));
 
 	itx_t *itx = zil_itx_create(TX_COMMIT, sizeof (lr_t));
 	itx->itx_sync = B_TRUE;