diff --git a/netwerk/cache/nsCacheService.cpp b/netwerk/cache/nsCacheService.cpp
index 1d594dce9b2b..e7c4acfa226b 100644
--- a/netwerk/cache/nsCacheService.cpp
+++ b/netwerk/cache/nsCacheService.cpp
@@ -75,6 +75,7 @@
 #include "mozilla/Util.h" // for DebugOnly
 #include "mozilla/Services.h"
 #include "mozilla/Telemetry.h"
+#include "nsITimer.h"
 
 #include "mozilla/FunctionTimer.h"
 
@@ -218,6 +219,20 @@ private:
 
 NS_IMPL_THREADSAFE_ISUPPORTS1(nsCacheProfilePrefObserver, nsIObserver)
 
+class nsSetDiskSmartSizeCallback : public nsITimerCallback
+{
+public:
+    NS_DECL_ISUPPORTS
+
+    NS_IMETHOD Notify(nsITimer* aTimer) {
+        nsCacheService::gService->SetDiskSmartSize(true);
+        NS_RELEASE(aTimer);
+        return NS_OK;
+    }
+};
+
+NS_IMPL_THREADSAFE_ISUPPORTS1(nsSetDiskSmartSizeCallback, nsITimerCallback)
+
 // Runnable sent to main thread after the cache IO thread calculates available
 // disk space, so that there is no race in setting mDiskCacheCapacity.
 class nsSetSmartSizeEvent: public nsRunnable 
@@ -452,7 +467,7 @@ nsCacheProfilePrefObserver::Observe(nsISupports *     subject,
                 return rv;
             PRInt32 newCapacity = 0;
             if (smartSizeEnabled) {
-                nsCacheService::SetDiskSmartSize();
+                nsCacheService::SetDiskSmartSize(false);
             } else {
                 // Smart sizing switched off: use user specified size
                 rv = branch->GetIntPref(DISK_CACHE_CAPACITY_PREF, &newCapacity);
@@ -1447,8 +1462,19 @@ nsCacheService::CreateDiskDevice()
         mDiskDevice = nsnull;
     }
 
-    SetDiskSmartSize_Locked(true);
+    // Disk device is usually created during the startup. Delay smart size
+    // calculation to avoid possible massive IO caused by eviction of entries
+    // in case the new smart size is smaller than current cache usage.
+    nsCOMPtr<nsITimer> timer = do_CreateInstance("@mozilla.org/timer;1", &rv);
+    if (NS_FAILED(rv))
+        return rv;
 
+    rv = timer->InitWithCallback(new nsSetDiskSmartSizeCallback(), 1000*60*3,
+                                 nsITimer::TYPE_ONE_SHOT);
+    if (NS_FAILED(rv))
+        return rv;
+
+    timer.forget();
     return rv;
 }
 
@@ -2646,13 +2672,13 @@ nsCacheService::OnEnterExitPrivateBrowsing()
 }
 
 nsresult
-nsCacheService::SetDiskSmartSize()
+nsCacheService::SetDiskSmartSize(bool checkPref)
 {
     nsCacheServiceAutoLock lock;
 
     if (!gService) return NS_ERROR_NOT_AVAILABLE;
 
-    return gService->SetDiskSmartSize_Locked(false);
+    return gService->SetDiskSmartSize_Locked(checkPref);
 }
 
 nsresult
diff --git a/netwerk/cache/nsCacheService.h b/netwerk/cache/nsCacheService.h
index a50c264479b7..755e88a9f51c 100644
--- a/netwerk/cache/nsCacheService.h
+++ b/netwerk/cache/nsCacheService.h
@@ -182,7 +182,7 @@ public:
     static void      OnEnterExitPrivateBrowsing();
 
     // Starts smart cache size computation if disk device is available
-    static nsresult  SetDiskSmartSize();
+    static nsresult  SetDiskSmartSize(bool checkPref);
 
     nsresult         Init();
     void             Shutdown();
@@ -196,6 +196,7 @@ private:
     friend class nsProcessRequestEvent;
     friend class nsSetSmartSizeEvent;
     friend class nsBlockOnCacheThreadEvent;
+    friend class nsSetDiskSmartSizeCallback;
 
     /**
      * Internal Methods
diff --git a/netwerk/cache/nsDiskCacheDevice.cpp b/netwerk/cache/nsDiskCacheDevice.cpp
index d37044ba1ecd..8fb44eb3906b 100644
--- a/netwerk/cache/nsDiskCacheDevice.cpp
+++ b/netwerk/cache/nsDiskCacheDevice.cpp
@@ -118,6 +118,22 @@ private:
     nsDiskCacheBinding *mBinding;
 };
 
+class nsEvictDiskCacheEntriesEvent : public nsRunnable {
+public:
+    nsEvictDiskCacheEntriesEvent(nsDiskCacheDevice *device)
+        : mDevice(device) {}
+
+    NS_IMETHOD Run()
+    {
+        nsCacheServiceAutoLock lock;
+        mDevice->EvictDiskCacheEntries(mDevice->mCacheCapacity);
+        return NS_OK;
+    }
+
+private:
+    nsDiskCacheDevice *mDevice;
+};
+
 /******************************************************************************
  *  nsDiskCacheEvictor
  *
@@ -1120,8 +1136,14 @@ nsDiskCacheDevice::SetCapacity(PRUint32  capacity)
     // Units are KiB's
     mCacheCapacity = capacity;
     if (Initialized()) {
-        // start evicting entries if the new size is smaller!
-        EvictDiskCacheEntries(mCacheCapacity);
+        if (NS_IsMainThread()) {
+            // Do not evict entries on the main thread
+            nsCacheService::DispatchToCacheIOThread(
+                new nsEvictDiskCacheEntriesEvent(this));
+        } else {
+            // start evicting entries if the new size is smaller!
+            EvictDiskCacheEntries(mCacheCapacity);
+        }
     }
     // Let cache map know of the new capacity
     mCacheMap.NotifyCapacityChange(capacity);
diff --git a/netwerk/cache/nsDiskCacheDevice.h b/netwerk/cache/nsDiskCacheDevice.h
index 8d544863ab5f..f07adea08599 100644
--- a/netwerk/cache/nsDiskCacheDevice.h
+++ b/netwerk/cache/nsDiskCacheDevice.h
@@ -107,6 +107,7 @@ public:
     
 private:    
     friend class nsDiskCacheDeviceDeactivateEntryEvent;
+    friend class nsEvictDiskCacheEntriesEvent;
     /**
      *  Private methods
      */