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Index: sys/kern/subr_disk.c
===================================================================
--- sys/kern/subr_disk.c
+++ sys/kern/subr_disk.c
@@ -21,8 +21,13 @@
#include <sys/bio.h>
#include <sys/conf.h>
#include <sys/disk.h>
+#include <sys/sysctl.h>
#include <geom/geom_disk.h>
+static int bioq_batchsize = 128;
+SYSCTL_INT(_debug, OID_AUTO, bioq_batchsize, CTLFLAG_RW,
+ &bioq_batchsize, 0, "BIOQ batch size");
+
/*-
* Disk error is the preface to plaintive error messages
* about failing disk transfers. It prints messages of the form
@@ -150,6 +155,7 @@
TAILQ_INIT(&head->queue);
head->last_offset = 0;
head->insert_point = NULL;
+ head->batched = 0;
}
void
@@ -188,6 +194,7 @@
{
TAILQ_INSERT_TAIL(&head->queue, bp, bio_queue);
+ head->batched = 0;
head->insert_point = bp;
head->last_offset = bp->bio_offset;
}
@@ -246,6 +253,16 @@
return;
}
+ /*
+ * Impose a maximum number of passengers in any given
+ * elevator car. This limits the maximum latency for any
+ * given request without signficantly affecting the average.
+ */
+ if (bioq_batchsize > 0 && head->batched > bioq_batchsize) {
+ bioq_insert_tail(head, bp);
+ return;
+ }
+
prev = NULL;
key = bioq_bio_key(head, bp);
cur = TAILQ_FIRST(&head->queue);
@@ -264,4 +281,6 @@
TAILQ_INSERT_HEAD(&head->queue, bp, bio_queue);
else
TAILQ_INSERT_AFTER(&head->queue, prev, bp, bio_queue);
+
+ head->batched++;
}
Index: sys/sys/bio.h
===================================================================
--- sys/sys/bio.h
+++ sys/sys/bio.h
@@ -129,6 +129,7 @@
TAILQ_HEAD(bio_queue, bio) queue;
off_t last_offset;
struct bio *insert_point;
+ int batched;
};
extern struct vm_map *bio_transient_map;

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