libata: add comment documenting PIO latency issues on UP
Based on:
http://lkml.indiana.edu/hypermail/linux/kernel/0908.2/01420.html
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index dc72690..22ff51b 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -6616,6 +6616,13 @@
{
ata_parse_force_param();
+ /*
+ * FIXME: In UP case, there is only one workqueue thread and if you
+ * have more than one PIO device, latency is bloody awful, with
+ * occasional multi-second "hiccups" as one PIO device waits for
+ * another. It's an ugly wart that users DO occasionally complain
+ * about; luckily most users have at most one PIO polled device.
+ */
ata_wq = create_workqueue("ata");
if (!ata_wq)
goto free_force_tbl;